A321-Neo pilot reported being unable to pressurize aircraft during climb and returned to departure airport. Previous maintenance on aircraft for a vent avionics fault was done improperly; which caused the issue.

Narrative

During after takeoff flow I noted that the aircraft did not seem to be pressurizing normally. By the time we reached 10;000 it was not pressurized. A little over 11;000 we descended to 10;000. We entered a hold; called Maintenance Control/Dispatch. Were told to return to ZZZ. I [requested priority handling] with ATC; we were going to be an overweight landing. Spent some time holding to set up for the overweight landing. Asked for the trucks; to check for over heated brakes. The landing on XXL was uneventful; the brakes stayed below 300 degrees and we taxied to the gate.Cause: The trip began with a return to gate for a vent avionics fault; it was put on MEL. When we returned to ZZZ a mechanic supervisor came on and informed us that the proper procedures had not been followed for the MEL and the intake and outflow valves were open; we never going to be able to pressurize. This is disturbing to think about; an error was made and put an aircraft with over 200 SOB (Souls On Board) in a less than ideal situation. A HUGE mitigating factor was that I completed training just a week ago and one of the training spots was an overweight landing. I was well prepared for this event. Kudos to the training department!