I think the best thing to do here is call Sprint concerning your problem unlocking your LG Tribute L660P, surely they will be able to unlock it, but you may have to pay.

You could also call into one of their retail stores where they should be able to either unlock it on the premises or send it off to be unlocked. It shouldn't be a difficult thing to do though.
Failing that, I guess your other option is to sell it on. You could put it on eBay and although you would need to be honest that it is locked to Sprint, this really shouldn't be a problem as I have sold many phones locked to networks and have not had a problem.
When the phone sells, you could then look at either buying one on the network you want to join or buy one that is factory unlocked.