Problems with ABM - Apple Business Manager

We have the apple business manager in use and the sign in with AppleID isn´t possible, so we created manually an account with the mail address but still we can not purchase or activate the premium version afterwards. There is some bug in the purchase procedure as you can see here

Does anyone have a solution for using the premium version with ABM?

Dear @reneroemer,

We’ll check with @Tristan, I’m not familiar with ABM.
We come back to you asap

Thanks in advance.