Yep. That is not enough. You could run a most basic system on less than that even, but not if you do anything even slightly advanced, install any apps... really, the Minimum requirements are almost impishly difficult to reach. 
I would highly recommend a minimum 64gig stick for the function you are using. Error logs that accrue over time in Root would be enough to make a small USB drive wobble.
As @Aravisian already told you, it is not large enough.
Actually now I remembered my daughter-in-law had a similar problem when she was out of space in Mint Linux. As a beginner Linux user, she setup DejaDup to backup on the internal drive and she ran out of space in a few weeks. Never underestimate novice users 
If you really must stick to this 16GB of space, I have some suggestions.
Puppy Linux could be another possibility, but I did not find it very practical for a day to day computing.
[edit]
Just found this article.
Indeed "insufficient space" is the number one cause of login loop.
I tried everything on this thread including the loopback problem on Linux Mint, which I had on my LM install on a different partion(had 25GB of junk which I removed with bleachbit). Now back to my Zorin issue:
Latitude E5400 Intel Core 2 Duo P8700(2)@ 2.535G Intel Mobile 4 Series Chipset 1383MiB/3891MiB
Zorin Lite installations:
After 3 installation attempts I am resigned to abandoning Zorin. My password is rejected a few days after installation, first on a generic 16GB stick, next a Sandisk 16GB, and finally an 80GB Western Digital external hard drive. I had no problem setting up Zorin, adding or uninstalling software, downloading updates and files, until about the 4th reboot. Then my user name greets me in the center of the screen, and demands a password, which it rejects. I selected auto-login in the setup, so I don't know why I need to enter a password, an 8-character strong password by the way, which I have written down and memorized. So the problem is not me.
Last year I used Zorin on one of 4 partitions on my hard drive, and had no problems and rated it superb. I needed the drive space for other distros I was sampling, so I wrote over it with the intention of installing Zorin on a USB stick as a backup if the hard drive dies. This is where I am now. I had Manjaro on the WD80gb, and was able to install Manjaro to USB stick, so recently I tried to overwrite the WD80gb with Zorin.
Updates reminder wanted to install Firefox language packs, which proves artifical intelligence lacks artificial common sense, because I uninstalled Firefox in my first session. Later the shutdown menu disappeared, so I hung in Limbo, a bad sign which proved correct. Now it is a brick.
I tried some fixes I found, such as ctrl+alt+f1 to get into a text-only screen to enter the password, with no luck. It has no problem remembering my user name,but can't remember my password. I even booted into generic recovery, connected to internet via mtp and upgraded packages while removing others. Around and around and around. I tried shell prompt root with no result.
I used a password cracker, which found my user name, allowed me to change the password, stated the password was changed succesfully, but still no go.
I find it hard to believe I have not seen this problem on search engines, which is why I signed up here. I just have to know. I suspect uninstalling Firefox and installing Brave just scrambles its brain. In the prehistoric days of early Windows, my friends and I dealt with the blue screen of death with the saying " We put something in there that it just doesn't like". Can you tell me what Zorin does not like?
I have seen a few reports on here of problems when users have chosen auto login instead of standard password login. Maybe worth a search on here for "auto login" posts. 
However, 16GB USB is not good, even though I have that space for my /root partition, I do have to keep it regularly cleaned out
...finally an 80GB Western Digital external hard drive
Did you run a health check of that drive before installation?
See for example:
Especially these parts;
HDD has armature damage or too many bad sectors. Windows operates by throwing down new data in any available space (Which is why you must defrag); but Linux allocates Set Blocks to properly contain data.
Too many bad or damaged sectors can cause this to be next to impossible, excluding the HDD from Linux.
I had Manjaro on the WD80gb, and was able to install Manjaro to USB stick, so recently I tried to overwrite the WD80gb with Zorin
Thanks for this info. So the faulty HDD is excluded from the possible causes.
Or that Manjaro has some sort of magic touch to be able to run on a less than optimal disk? Since I do not have much experience in Arch based OS, it is just an speculation.
I got locked out on the 4th reboot on 3 different devices.
and that happened after Firefox crash?
Just remembered I did have such problem with Firefox and language pack.
I never used Firefox, uninstalled it, yet received update notifications for locale and language packs. What's with that?
Thanks for this additional information.
Have you ever considered removing unnecessary language packs completely using Bleachbit?
my LM install on a different partion(had 25GB of junk which I removed with bleachbit
I think you might want to try the same after the fresh installation of Zorin.
In fact, my husband fed-up with this instability of Firefox and switched to Vivaldi just last month. Considering he had been a faithful Firefox user since his Motorolla iBook era, it was indeed a major move.
On Zorin and every other distro I use I never used Firefox, uninstalled it, yet received update notifications for locale and language packs. Why do you think Zorin locked me out 3 times?
Difficult to say.
I myself is (still) a Firefox user.
What I noticed was that after this (controversial) Proton interface update (ver. 89) I had a stability issue. It was so bad and at one point, it froze entire Linux OS just like BSOD in Windows.
I wrote back the Clonezilla image from the previous month (which had no language pack BTW) but after I manually installed the language pack (Japanese), instability came back. I finally let Zorin update to add language pack automatically and everything seems OK now.
In any case, I think it would not hurt to remove unnecessary language pack with Bleachbit immediately after fresh installation before giving up.
Actually, it ran pretty fast and the main reason for low space was i installed too much garbage. I had 6 GB of free space when it was newly installed. I think I'll just simply reinstall
Also lmao, I missed a lot, gosh lots of replies
I'll check those, but i really really like the UI of Zorin
Btw, also found something called "Tiny7" which is basically 1 GB version of Windows 7, it was made by stripping down all unnecessary things. I might check that too