Just had a thought (dangerous I know!). I am wondering if it is not taking because the machine will be identified as Zorin not Ubuntu 20.04 ("focal fossa"). Think I posted something recently but can't remember the procedure to temporarily change the machines id to a different OS then back again once you have installed the software. Bear with.
Update 13:50 GMT - Open a terminal and enter the following (the ones in bold), one at a time:
Oops - Markdown keeps the ** instead of showing the key elements of bold!
zorin@zorin:~$ **cd ..**
zorin@zorin:/home$ **cd ..**
zorin@zorin:/$ **cd etc**
zorin@zorin:/etc$ **sudo nano lsb-release**
You will get a screen up like this:
Change Distrib_ID to Ubuntu
Change Distrib_Release to 20.04
Leave codename as is.
Change description to "Ubuntu 20.04 LTS"
To save changes in nano editor, press Ctrl+ O to write out the new changes. You will be asked if you want to overwrite - say yes.
Then Ctrl+ X to exit.
Once you have made the changes, try installing again and if successful, remember to reverse the procedure so you get updates to Zorin!