Rule must begin with a letter

What causes this issue?

You have attempted to create a subrule with a name that does not begin
with a standard alphabetic character. The first digit may not be
numeric. Only characters belonging to the international character set
are supported.
'ABCDEFGHIJKLMNOPQRSTUVWYXYZ'

System Response

None

How to fix this error?

Choose a different name.

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.