Chosen Solution
There are a few possibilities here! Are you sure you fed the ribbon cable into the logic board connector correctly and fully? Did you flip the latch down so the cable is compressed? Review this guide MacBook Pro 13" Unibody Mid 2009 Logic Board Replacement Step 12. I find a piece of electricians tape can help in guiding the cable in using it as a pull tab. Make sure you don’t damage the latch? Are you sure you got the correct keyboard for your system? MacBook Pro Unibody (A1278) Keyboard
