I’ve run into the same issue and found out there are other people getting the same error on different software. And there is a workaround on youtube. You could try to open the terminal and then type “sudo spctl --master-disable”
This will add an additional option when you go to “Settings > Security & Privacy > Allow apps downloaded from”. You have to check “Anywhere”. Then it should work.
Another workaround was suggested by @IvanYossi:
"first test there is a quarantine flag typing in terminal xattr /Applications/krita.app and hit enter. the results will return “com.apple.quarantine” and maybe other results. This line is a flag and tells macos to check for signature as the file comes from the internet.
to remove the flag type: xattr -d com.apple.quarantine /Applications/krita.app you can check the flag is removed by typing the first command and see the quaratine flag gone."
Hope it helps!