Customer number &1 does not exist

What causes this issue?

You entered a number for a customer that does not exist in the system.

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

System Response

How to fix this error?

Check your entry. If you made the wrong entry by mistake, overtype the
field with correct number. If you do not know the correct number,
select the desired customer number using the possible entries function
to help you.

Error message extract from SAP system. Copyright SAP SE.