Can anybody please tell me why I am getting this error from a update downloaded from the Graphisoft Website. Even allowing it in System Preferences doesn't work
I don’t think AC20 is compatible with Big Sur. Apple introduced strict app signing.
there may be a workaround.
This is for AC21 on Catalina but may help fix your issue but it sounds like you may have already tried this.
Installed AC 20 from Disc without any problems and works well but I can't update it to 8005. Tried most of those suggestions but none worked. Seems to use the Java in the installer of 8005 instead of the installed one on the computer.
I can only assume that it is not supported in Big Sur. There may be a workaround if you can contact local support.