Check numbers require a numerical portion

What causes this issue?

A check number consists of a numerical sequence of digits, or if need
be it can also contain a non-numerical prefix (for example, CH000014)
or a suffix (for example, 000014X). If the numerical part is missing,
the the system cannot work with the check number.

How to fix this error?

Check the way the check number has been written.

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.