Changing the tax key in the outgoing invoicing requires the new authorization FAKSTEUERSCHLUESSEL.

If this authorization has not been assigned to a user in basic data ADMIN →  AUTHORIZATION PROFILES →  Assign user rights, this authorization will not be checked!

If the authorization FAKSTEUERSCHLUESSEL is used, either Create OR Edit must be activated so that the user can modify the tax key.

If the user is not supposed to be authorized to modify the tax key,  Create AND Edit must be deactivated.
In this case, the field Tax key in the billing lines is blocked.
This field is also blocked in the billing lines if a user does not have the authorization FAKSTEUERSCHLUESSEL.