2015 MBP totally dead after new battery replacement

Bought an Ifixit battery to replace a puffy one in 2015 MBP. MagSafe lamp worked fine before removal of old batt (new charger btw). Carefully installed new batt, replaced back cover on laptop, plugged in adapter and the green/amber lamp doesn’t light up. Had professional look it over, said everything looked normal, he took out and reinstalled, tried all the key combinations to reset the systems but still no lamp. Metered where adapter plugged in, had normal voltage coming in at insertion point so MagSafe seems fine. He then took out the new battery and tried again with old one, but still no lamp. Did I trip a safety feature somewhere during installation or is it toast? No smoke or sparks seen. I’m told this might be the only model that won’t power up without a battery connected, not sure. Thank you!