Sender field & has length &, but receiver field & has &

What causes this issue?

The sender field does not have the same length as the receiver field.
If the sender is longer than the receiver, information could be lost
during the field movement.

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?

Error message extract from SAP system. Copyright SAP SE.