With the release of Cubase 12, Steinberg has dropped support for the VST format in favor of the VST3 format. This change only applies when using Cubase 12 natively.
If you have previous Cubase sessions in which you've used the VST format instead of VST3, Cubase will not load these plugins running natively.
One of two solutions are available:
Run Cubase 12 using Rosetta
Interestingly, the VST format is still able to be used using Rosetta 2 translation. You can run Cubase 12 using Rosetta by locating Cubase 12 in the Applications folder, right clicking, selecting "Get Info" then check "Open using Rosetta" checkbox so the application runs on the Intel compatibility layer:
Use the VST3 format instead
While not the easiest solution, this is a much more permanent solution, as all VST support is expected to be abandoned in the future. Unfortunately, Steinberg does not offer an easy VST to VST3 format migration tool for existing sessions.
Note: FG-X is not available in VST3 format. To use the original version of FG-X in Cubase 12, Rosetta is required. We encourage replacing FG-X with the newer FG-X 2.