I have a storage unit in my name. Can I sell the property that belongs to a friend. The friend owes me over $1200.

Sure if that friend gives you permission to do that ( I would get it in writing)
Or if you sue your friend, get a judgement and then attach the personal property to pay for the debt.

Conversely, if you don't win a case or get permission, the friend can sue YOU, even if he or she still owes you 1200.