Note: you are making changes within the reserved name range

What causes this issue?

Names that do not begin with a letter are intended for delivery from
SAP.

System Response

How to fix this error?

If you do not intend to change the delivery object and have only
entered a name by mistake that begins with a letter, you should end the
editing without saving.
If you really wanted to carry out a change, you should note that you
are dealing with a delivery object. In this case, carry out all
relevant measures (change in Customizing clients, release info, etc.).

Error message extract from SAP system. Copyright SAP SE.