Function code may not begin with '/'

What causes this issue?

Character '/' has a special meaning.
At runtime, entries in the OK-Code field of a screen beginning with '/'
are interpreted as system codes and are therefore not edited by the
program.

System Response

The system does not assign function codes beginning with '/'.

How to fix this error?

Choose another function code.

Procedure for System Administrators

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.