Try to disconnect the cable and ride it. Does it slip?

If not, then it is a cable/indicator chain/shifter issue.

Otherwise, follow the excellent advice by cny-bikeman.

The spring can be bought for less than $1 from Niagra.

Since you just got the bike, you may not know if the hub had been messed by others. I have bought several bikes and the hubs had been taken apart but was not put back together properly (parts out of order).

On the top of the spring, there should be a spring cap to even up the spring force against the thruster washer on one end the the right cone on the other. Some model does not use the washer but the thruster is of different design.