You are not permitted to use result type &1 to build formulas

What causes this issue?

You want to create an expression of the expression type <ZK>SAP Formula
Interpreter</> (implementing class 0FB001), but the result type is not
permitted for this expression type.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Use the input help of the <ZK>Result Type</> field to select another
result type.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.