How can I set a constraint on a value of a computed field?

I have set a field "Functional Value" with the following computation : aggregate(Satisfies,sum(Type="System Requirement" AND IsLive()?"System Functional Allocation":0))

I would set a constraint on a picklist field according to the value returned by the Functional Value field.

But unfortunatly, I have the following error message : "Unable to evaluate source relationship based on field "Functional Value" because its computation involves an external information function"