Technical error in POR subscriber number

What causes this issue?

The POR subscriber number was entered incorrectly. The following errors
are possible:

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

The POR subscriber number is too long. The program expected the short
number (5 characters).
The check digit, that is, the 5th character of the short subscriber
number, is missing.
The short POR subscriber number contains hyphens '-' or blanks ' '.

System Response

How to fix this error?

Check the entry for the above-mentioned sources of error.

Error message extract from SAP system. Copyright SAP SE.