Last gear does not work
First, it's not clear which gear you mean by &quot;12th&quot;.  Is it the largest or smallest one?
It sounds like your rear derailleur is not properly adjusted. As 11 of your 12 gears apparently work fine, it's most likely a problem with the limit screw for derailleur.  It's probably not allowing the derailleur to travel far enough.
Park Tool has quite a few videos covering how to perform most bicycle-related maintenance.
In this case, Rear Derailleur Adjustment is the appropriate video.