Have upgraded to BigSur and then tried to install Nik Collection 3 but installer wants Mac OS 7 or newer?!!
PRH
Hi, Peter. Several users have already opened topics on the forum to ask about this. Be assured that DxO knows about the problem. 
Apparently they fixed the installation problem and it should install on Big Sur without any problems. I just installed it now. The release notes now include Big Sur. Download it again