Meanings occur in context.
I am ill so I cannot join you tomorrow or will not be able to join you tomorrow. [either can be used, cannot can refer to a time in the future].
Question: Can you attend the meeting tomorrow?
Answer: Yes, I can [my schedule allows it or health allow it.] but I'm afraid I won't. [will of intention] I never attend meetings at your office.
Question: I know you are ill. Will you be able to join us tomorrow? [Does your physical state or other state allow you to join us? Specifically. ]
Answer: I will not be able to join you tomorrow. [am physically unable to do so then]
Question: Will you open the window?
Answer: Yes, I will. [will used in asking someone to do something].
Question: Will you attend the meeting tomorrow?
I would go if I could, but I simply cannot go because I am unable to but I will go in the future, I promise.
[parse: conditional, conditional of can, negative reference to a future and future intention and physical incapacity.] The cannot here is: it is not possible for me to go due to a physical or other situation that stops me from going.