Maximum number of users for server module &1 exceeded

What causes this issue?

Only a limited number of users are permitted to use the SNP optimizer.

System Response

The system cannot execute optimization.

How to fix this error?

Check the number of existing users using transaction /SAPAPO/OPT03.

Procedure for System Administrators

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.