open /Applications folder (select Open “Applications” when navigating in the dock and right clicking).
Right click-hold on Nuendo 12.app and choose “Show Package Contents” from the context menu.
Navigate to /Applications/Nuendo 12.app/Contents/VST3
Rename anymixPro.vst3 to anymixPro.vst3_ or similar (make sure to change the extension)
You get prompted first whether you want to change the extension and then for elevation with touch-id or password.
As in @Durcu 's solution, you can open Nuendo 11.app 's package content in the same way and COPY (cmd-c cmd-v or option-drag) the old anymixPro.vst3 over from there.