Can't change into the largest sprocket on rear derailleur
Just about any new bike you ever purchase will need a good tune up after the first 100 miles, not unlike a lot of cars or motorcycles. It's perfectly normal for most of the parts, especially parts under tension like brakes and cables, to settle and loosen a bit the first few times you ride the bike.

It's likely that the cable for the rear shifter has loosened a little bit and needs to be tightened, or the limit screw has settled. The shop you bought the bike from should have no problem performing the tune up for you, it's very common practice ( and if they try to charge you for it, hesitate to give them your business, the 100 mile tune-up should be complimentary ).