A configurable material cannot be finished product unless it has a set of values for it's configuration characteristics which is why it is not possible to do a MTO directly for the configurable material.
In ECC if you use material variants you can use the material variant as MTS. You need to create a new material and make it as a configuration variant of a configurable material, you can do that at plant level or at material (client) level by assigning the configurable material to the material in material master.
In Basic Data 2 (client/material level)
In MRP 3
Once you do it the Variant check box which is greyed out will become checked.