How to delete old Mac OS update installations?

For one thing, how did an installer wind up in your User folder? That is what the file path shows at the bottom of your screenshot. The installer should be in the Applications folder. And, it appears you did have it installed at some point? You should not try to delete apps placed there by the system (and, in Big Sur, you may not be able to); if I remember correctly, the Catalina installer remained after the install (most OS versions installers delete themselves after the install is finished).

If you do not have either of these installer.dmg's in your HD > Applications folder, then the only way I know of is to go to recovery, erase your entire drive, and start over by reinstalling the OS. Make sure you have a full backup; however, be very selective as to what you want transferred to the Mac from the backup since yours are in your User folder which means they may be copied over again.

Hello Sana_Is_VIP,

Thank you for using Apple Support Communities! It sounds like you’re having a problem with deleting an older Mac OS download from your desktop. We would be happy to help! 

The information you have provided so far has been great. In order to ensure we understand what is happening, what is the exact message that you receive after you try deleting the file from the trash? 

If you haven’t so already, we encourage you to check out Delete files and folders on Mac

Cheers!

Sana_Is_VIP,

Thank you for sharing. If you still need further assistance please feel free to Contact Support.

Have a good day!