That should have unlocked your handler section - did you copy your applicationhost.config from another machine or an earlier backup?
In any case, you can run the below from an elevated command prompt to unlock the handlers section
%windir%\system32\inetsrv\appcmd.exe unlock config -section:system.webServer/handlers
Note that you may still hit other unexpected errors since it seems like your root configuration is not what it should be.