Enter one customer only

What causes this issue?

The document contains at least two items with different customer
accounts.
Offsetting items that you have entered manually (that is, not using the
clearing process) have to be posted to the account of the customer that
presented you with the bill of exchange or check. For these purposes,
the document must only contain one customer account.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.