It's not your vehicle to sell.
You would have to first either pay off the remainder of the existing loan or have someone assume the remainder of the loan
Unless you have the cash on hand to pay the difference, you can't. You and millions of other people are in the same boat because you failed to "crunch the numbers" regarding depreciation. Banks shamelessly make car loans to people who cannot really afford them. So, either get a better paying job or a second job, or start selling off your possessions and reducing your frivolous expenditures so that you can dig yourself out of the hole you're in.
You have to pay the difference in order to get a clear title to transfer.
You sell...then you pay the difference
It's not your car, so you can't. To make the deal, you need to be able to get the title from the lender. For example, let's say the car is worth 8,000 dollars and you owe 10,000 dollars on it.
If you can sell it for 8k, you need to come up with the other 2k to make the deal.
You find a way to pay off the difference
You can't sell what you don't own.