No, I mean... Open Software and check the updates as shown in the O.P. ^
No, the error message is different. 
Try
sudo apt install --reinstall ca-certificates policykit-1-gnome
It's showing the same error again and again
sudo apt install --reinstall ca-certificates policykit-1-gnome Reading package lists... Done Building dependency tree Reading state information... Done The following NEW packages will be installed: policykit-1-gnome 0 upgraded, 1 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded. Need to get 170 kB of archives. After this operation, 152 kB of additional disk space will be used. Get:1 http://ubuntu.mirror.snu.edu.in/ubuntu focal-updates/main amd64 ca-certificates all 20210119~20.04.2 [145 kB] Get:2 http://ubuntu.mirror.snu.edu.in/ubuntu focal/universe amd64 policykit-1-gnome amd64 0.105-7ubuntu2 [25.0 kB] Fetched 170 kB in 0s (351 kB/s) Preconfiguring packages ... (Reading database ... 288864 files and directories currently installed.) Preparing to unpack .../ca-certificates_20210119~20.04.2_all.deb ... Unpacking ca-certificates (20210119~20.04.2) over (20210119~20.04.2) ... Selecting previously unselected package policykit-1-gnome. Preparing to unpack .../policykit-1-gnome_0.105-7ubuntu2_amd64.deb ... Unpacking policykit-1-gnome (0.105-7ubuntu2) ... Setting up ca-certificates (20210119~20.04.2) ... Updating certificates in /etc/ssl/certs... 0 added, 0 removed; done. Setting up policykit-1-gnome (0.105-7ubuntu2) ... Processing triggers for man-db (2.9.1-1) ... Processing triggers for ca-certificates (20210119~20.04.2) ... Updating certificates in /etc/ssl/certs... 0 added, 0 removed; done. Running hooks in /etc/ca-certificates/update.d... done.
Log out and in, then check.
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killall gnome-software
killall nautilus
Relaunch Software and check...
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killall nautilus nautilus: no process found
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oh... Are you on Zorin Lite?
Or was nautilus just not running at all...?
No, Zorin 16 Core
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What is Nautilus?
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File and desktop Manager for Gnome Desktop.
The file manager is just called "Files" in the app menu because Gnome wants to be Windows when it grows up.
Okay...
After installing the above packages, the gnome-software app needed to clear the cache and completely re initialize. I thought logging out and in would be enough to do that - I was wrong on that. The killall command on gnome-software finally got it to refresh.
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Yes, U are right.
Thanks for giving your precious time & effort. It's really appreciated.
Have a great day ahead.
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I hope GNOME fix their applications as soon as possible
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Had a similar issue with Zorin OS 16 Core Edition. The steps in this thread resolved the issue and appears to be working just fine. I did notice that Nautilus is not installed. I don't think it is necessary to install unless you like that type of file manager. Thanks to Aravisian for the assist.
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Nautilus is not installed on Zorin OS LITE, so if you are using that, it comes with Thunar File manager, not Nautilus.