As Ken points out, you cannot upgrade from 2.6 straight to 3.3, you must go via 2.7 first.
I would also recommend visiting http://s2g2cwts.inleap.net/onlinetraining/admin after you've put the code in place, or better still perform the upgrade via CLI.
It appears that you have not run the database upgrade either.
The 'requiredbytheme' column was added in Moodle 3.2 via MDL-56902. If you are still seeing reference to this column, even after switching to the 2.7 codebase, it suggests that you have some of the 3.2 codebase still hanging around.
I would recommend moving your existing installation to one side, and placing a fresh download of Moodle 2.7 in its place, copy of the config.phpl and download any updates for any of your plugins.