MacBook Pro 15” Mid 2012
I had the exact same symptom upon initial completion. The keyboard ribbon is actually 2 pieces on mine. There is a plastic cover sheet (that I tried to use as the ribbon the first time) and then there is the actual ribbon sheet. The keyboard works perfectly now that I have the actual ribbon sheet inserted into the the ZIF connector.
It took entirely too long. I was being very careful. Recovery of the keyboard took me less than an hour. I don’t want to admit how long the original work took me.
I hope you already have yours working. I was very careful due to not having an alternate device for my son to use if the work had failed. The other likely point of failure for the keyboard is damage to the ribbon as it goes under the battery pack. I caught myself almost poking too hard on that spot.