Error when adding Posting Lines for non-PO / Non-Receipt Manual Supplier Invoice

Dear Community,

I am facing an issue while adding posting lines for a manual supplier invoice that is not PO-based and does not involve any receipt.

Whenever I try to add the posting lines, I get the following error (please refer to the attached screenshot):

"ORA-20110: PostingProposalHandling.NEWNOTALLOW: Creation of Posting lines is not allowed."

I would appreciate it if someone could guide me on what could be causing this and how to resolve it.

Thank you in advance for your support!