Last gear does not work My bike has 12 gears on the rear wheel. When I put the last gear (12) it starts constantly changing from 12 to 11 gear without me doing anything. For example, I put the last gear and it works; right away it changes to 11 gear for a very short time; then back to the last gear and so on. All the other gears work fine. I don't known what's possibly wrong. Sorry for my English.
Thank you!
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.