Can I say "I'm not going to be able to."? Is that a complete sentence? Or does it always have to follow with a verb?
Your sentence "I'm not going to be able to" is correct and should be understandable, as long as there is some prior question or context asking whether or not you are able to do something.
However, Your sentence may sound a bit odd. You should still add a verb at the end. I'd suggest you use one of the following alternatives:
- I won't be able to.
- I will be unable to [verb].
- I'm going to be unable to do that.
- I'm going to be unable to do so.
- I'm not gonna be able to.
- I'm gonna be unable to.