I was charged in error with U.S. withholding tax when in fact, I had Capital loss in selling U.S. listed partnership stocks. Whom/where to get refund?

Assuming you are a nonresident alien and the taxes were paid to the US government that is who you apply for a refund with. You would need to file a 1040-NR (Nonresident) to claim any refund due.   TurboTax does not handle 1040-NR filing but you could contact one of our partners at Sprintax for assistance with that filing.

Here is an IRS link with more information on Nonresident Alien withholding.