The default value for variable &1[&2] must not be '#'

What causes this issue?

Variable &V1&[&V2&] has a ddefault value &V4&. But this value is initial
or equal to '#'. For variable type (VARTYP = '&V3&' ) this is not
permitted.

System Response

Variable &V2& cannot be used.

How to fix this error?

Check variable &V2&.

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.