Inference for characteristic variable &2 (facet &1) not possible

What causes this issue?

The syntax check recognized an error: the inference of &V1& required
under <ZH>INFERENCES:</> is not possible in any of the value assignment
alternative of the constraint.

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?

Please note that a variable in the conditional part of the constraint
(CONDITION:) cannot be inferred in this constraint. Also, a variable in
a conditional restriction (after IF) cannot be inferred in this
conditional restriction. The relevant declaration determines which
inferences are possible from which table or function calls.
Please note that formulae cannot be expanded after all variables.
Special criteria apply to restrictable variables.
Either remove &V1& from the list of required inferences, or change the
restriction part and/or the conditional part of the constraint
accordingly.
Before you try to restrict the characteristic variable under
INFERENCES, you should check in the characteristics maintenance
function (<DS>Restrictable</> indicator) whether this is allowed.

Error message extract from SAP system. Copyright SAP SE.