Can you have 了on the end?
I came here to ask the exact same thing!
Using 了 at the end indicates that 不行 is just starting to take place.
An example would be 这个（病人）不行了，which means this one/patient is not going to make it (3 minutes ago there was still hope that we can save this patient, but the condition of the patient got worse and there is nothing we can do now).
这个不行了 sounds like something i've said 100 times sucessfully
I think 了 just softens the expression and should be allowed.