Invalid name

What causes this issue?

Reasons why the name is not valid:
The name must start with a letter.
The name may only contain the following character types:
Letters
Numbers
Underlining
and can end with blanks.
The name HEADER may not be used.
The names of supplementary fields may not exceed 21 characters.

System Response

How to fix this error?

Enter a valid name.

Error message extract from SAP system. Copyright SAP SE.