Maintenance strategy & does not exist

What causes this issue?

You entered a maintenance strategy 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 the key entered. If you entered the wrong number by mistake,
overtype it with the correct value. If you do not know the correct
identifier for the maintenance strategy you want, you can use the
possible entries function to help you find it.

Error message extract from SAP system. Copyright SAP SE.