CRJ-550 flight crew reported unable to pressurize the aircraft after departure. Post flight inspection revealed a valve was improperly deactivated by maintenance.

Narrative

We arrived at the aircraft with multiple Deferred Maintenance Items. One of which was the Ground Valve. This was DMI'd (Deferred Maintenance Item) and placarded in accordance and the Flight Crew procedure was followed and verified before departure. During our take off roll out of ZZZ; as soon as we left the ground; we felt the pressurization of the aircraft start to climb as the CABIN ALTITUDE" Started to climb as well. After getting the gear up we deiced to clean up and level the aircraft; we went up another 1000' to check the pressurization and it confirmed our suspicion as the aircraft was not pressurizing and the Cabin Alt was the same as our pressure altitude. At that point I; the Captain (as Pilot Flying) took over the controls and Radios while I had my First Officer run the appropriate checklist. After deciding there was nothing to be done and the pressurization was UN-controllable we decided to return to ZZZ. We coordinated with ATC; the Flight Attendant; the passengers; and Operations. We set up for an approach and landing to ZZZ all according to standard operating procedure. We returned without incident to ZZZ; An emergency WAS NOT declared and we subsequently returned to the gate and deplaned the passengers. Latter we learned that the Ground Valve that was supposed to be deactivated in the "closed" position was in fact in the "open" position - contrary to the MEL Maintenance Procedure. There was no way of knowing the procedure was done correctly before departing for flight."