Tire and Tube size mismatch
If its unopened you might be able to take it back to the shop and swap for the right size, but the difference between 2.125" and 2.175" is 0.05" which is only 2.3 % smaller so it would probably work fine.

There may be a bit more tube than you strictly need, and this could make pinching the tube easier when fitting it, so do take care.

Otherwise, a tube is probably one of the cheapest parts on a bike, and buying another won't break your wallet.