Either your range adjustment is off, or your cable tension. Assuming your derailleur is not bent or damaged, this is likely what's wrong.
Your range adjustment screw that limits how far toward the 'big' gears you can shift needs to be loosened a little, probably as little as 1/4 turn counterclockwise.
Check your derailleur: you should have two screws, one labeled 'H' and one labeled 'L'. Try loosening 'L' about a quarter turn.
Another possible issue is the cable tension could be too low, possibly because the cable has slipped or stretched. Make sure the cable clamp that secures the derailleur cable to the derailleur is snugly tightened. There should be a barrel adjuster where the cable enters the derailleur, or on the shifter body on the handlebars. You might also try turning that counterclockwise 1/4 -1/2 turn.
Finally, make sure your chain is clean and oiled, as the friction can interfere with proper shifting.
Further reference: https://www.artscyclery.com/learningcenter/shimanorearderailleurs.html