The classes are not valid exits for inbound distribution

What causes this issue?

This message is output due to one of the following reasons:
No class exists with the specified name
The class exits but does not implement the required interface
IF_INBOUND_EXIT_BCS.

System Response

The invalid entries are highlighted in red.

How to fix this error?

Change or delete the invalid entries.

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.