Syntax error: &1

What causes this issue?

In Customizing, you can define a check function that checks new
specification keys entered, regardless of the specification type. This
function is available in the system and has rejected the specification
key.
Independently of this, the system also checks whether the specification
key contains the special characters <LB>*</> or <LB>+</>. These special
characters are not permitted for a specification key.

System Response

The system does not accept your entry.

How to fix this error?

Correct the specification key. Contact your system administrator if
necessary to determine the syntax with which specification keys of this
specification type must comply.

Error message extract from SAP system. Copyright SAP SE.