Learn English – say ‘get off that/the chair’

General answer to you and everyone else who asks the question Can I say X? without providing any context for saying X: You can say anything you want to anyone you want at any time you want and anywhere you want. The important question is Are you prepared to accept the consequences of what you want to say? Unless you are, don't say it.

Specific answer: If the chair has arms, then you should probably say "Get out of that chair". For a chair without arms, however, "Get off the chair" is standard. "Get off the couch" is correct.

None of those sentences is polite English, though. They're like direct orders in the military from a superior officer to an inferior officer or enlisted person. They indicate annoyance, impatience, anger, or some other negative feeling. They sound haughty and imperious.

It would be polite and probably not cause a fight if you said "Please get off the couch" or, even more polite, "May I ask you to get off the couch?" In both cases, however, you'll probably need to give a reason. The person you're asking to get off the furniture is going to ask why he or she should get off.

You'll get better answers if you ask better questions. Specifically, if you provide some context for your question. This one has no context beyond a disembodied sentence. We have no idea who you are (i.e., your role in the scenario) or who the sitter is or why you're supposed to ask the sitter to get off the couch. Maybe you work in a furniture store and want to ask a tired or lazy or sleeping customer to find a coffee shop or a hotel instead of sitting or sleeping on your merchandise?