I had the same issue. My MacBook Pro was charging the battery, but it wouldn’t boot (this was after I replaced the battery). Solution: I took the back cover off and rechecked each connection. This is what I found to be the cause of my issue: after removing the LEFT hinge cover, then go to step 29. remove the silver metal bracket. Under this bracket there are TWO connections. There is a small connection hidden under the large connection; the small connection was not connected, and this was what cause the issue. I connected it and my MacBook Pro is working fine with no issues whatsoever.