The ArchiCAD 8 keyplug should work just fine with ArchiCAD 7 also. It works here with no special installation at all. I can even have both of them open at the same time and do a copy/paste between them. Only if you want to run AC 6.5 or 6 you need some installation (it is on your AC 7 disc).
Maybe you need to do a new installation of AC 7?
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Gorazd Rajh
From AC 6.5 onward, i9-14900HX, 32 GB RAM, RTX 4060, Win 11