I just installed Nik 4.1.1. on an external ssd, from which my Mac runs macOS Big Sur. After the installation, I clicked on “Finish”, waited a while and tried to eject the installer. Instead of doing so, macOS presented this:
This seems to be a persistent bug in Big Sur. You can “Force Eject” the installer after trying to kill the task a couple of times. I’ve had this occur with Nik,DXO PL, and some other software. It has to do with the new security model. It might be that there is a bug in the installer packaging too. I’ve reported this multiple times to Apple, to DXO, and to other vendors whose installers have the same issue.