Do you still have this problem now?
I too am experiencing this issue and it is a bit frustrating. I'm trying to make a custom pack on forge 1.12.2 and I know there is valid BoP versions for 1.12
Posts by Bludacious
The Aternos forums are now deprecated
Learn more about this change here.
For help regarding Aternos, we recommend you to visit our support center.