Document &1 assigned to contract cannot be transfer posted

What causes this issue?

No contract was entered as a target specification for transfer posting.
Items assigned to contract could not be transfer posted.

System Response

Items assigned to contract were removed from transfer posting

How to fix this error?

If you want to transfer post the items assigned to the contract
account, you must enter a contract as a target specification.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.