In D365, once the users enable Use Posting Definition in the GL parameter, the Prepayment button becomes greyed out in the Purchase Order form.

This behaviour has been documented in the LCS link below:

https://fix.lcs.dynamics.com/Issue/Details?bugId=402569&dbType=3&qc=004a2a30cf207a7854d57aaf54dab252bcf7758685e67766d7792b11f8f02059

Our customers would like to request that this feature be improved to allow for the Prepayment button to be available regardless of the setup.