What if a student cannot pay the APC of a journal I am submitting my article to IEEE Access to get its rapid response (degree deadline time problem). I'm hopeful of their fee waiver/discount because I am student/unemployed as well as physically impaired. However, I am not sure if I would be definitely given the fee waiver/discount. 
What if I get acceptance of my paper from IEEE Access and find myself unable to pay the APC? Can the journal take any legal action against me?

Note: Application for fee waiver/discount to the journal cannot be lodged before submission/acceptance.
No, they won't/can't take legal action. The worst that could occur is that they won't accept the paper. Nothing to worry about beyond that. 

But if you are a student, explore with your university whether they have funds to pay the fees. Some will be able to do this and others not. If you are working with a professor perhaps s/he has grant money that will pay such fees also. It is worth asking. 

Not all journals will have liberal forgiveness policies, but some do. Just ask and be prepared to state your case.