The selected key is reserved for SAP

What causes this issue?

You tried to add a new entry and in doing so, used a key value that
lies ouside of the namespace you manage.
This may lead to problems in future upgrades.

System Response

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

How to fix this error?

Check your entry and if possible, enter a key value from within the
namespace you manage.
In the case of content repositories, all two-character names and all
names beginning with Y or Z are reserved for customers.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.