Sorry for the delay. I have just checked on my machine, and Scaler is registered properly in GarageBand:
However, I did not register it FROM GarageBand but another DAW.
Once Scaler is registered, it is valid on the whole system. You don’t have to register for each DAW you are using.
Could you try to launch Scaler from another DAW and register your license file from there? Then check in GarageBand if everything works properly.
Cheers,
Ed