Contract account &1 belongs to collective bill account &2

What causes this issue?

Contract account &V1& belongs to the collective invoice account &V2&.
Transfer posting is not permitted for contract accounts that belong to
a collective invoice.

System Response

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

How to fix this error?

Specify a valid contract account.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.