MacBook won’t turn on after battery replacement

The only things that comes to mind, since you already checked connectors multiple times, are the full logic board power drain to apply before starting the job and the low power applying needed after completing replacement. The first point is part of the ordinary replacement procedure but second one is due to a glitch in the logic board design. Overlooking one or both may damage the board, in which case a component level repair or logic board replacement might be needed.