MacBook beeps but won't turn on after logic board replacement.

I replaced a dead 12” MacBook 2015 with a working 12” MacBook 2015 bottom assembly with logic board and battery (had partial broken screen that would boot).

After replacement, I connected to an USB-C to USB cable and using a 5-Watt iPhone charger for low voltage charge, I let it charge for 20 minutes then proceeded to try to power it on, it didn’t start up,

I switched over to the regular charger and managed to get it to boot up for around 3 seconds everything booted, screen keyboard lights, then it went dark again.

Now all I get is the beep sound when I plug in the cable and I get a beep sound occasionally when I try different SMC reset. I’m planning on leaving it charging over night by low voltage and checking it out again tomorrow.

Is there any guidance or suggestions to try'?