The original problem I was dealing with was both sides of the sink were backing up when the dishwasher ran or the disposal was turned on. So, thinking there was a clog somewhere, I attempted to find it myself and clean it out. After not finding the clog, I decided to put the pipes back together and save it for another day. All went back together fine except top piece that attached to the basin. The nut spins only to a point and then it gets loose again. When you run water there is a leak. So - clog not fixed and now I have a leak. Any advice on either? Photo attached - hope it helps.

Are you talking about nut #1 or nut #2?

Assuming you are talking about #1, the threads of the nut are stripped or the nut is broken. Probably from over tightening. Replace. That piece is a 1 1/2" sink tail pipe. Will have to cut length of new one to fit.