My 2012 MacBook Pro won’t turn on, the battery is dead. The charger shows green. Also removed battery. Tried to turn on MacBook plugged in without battery. Still don’t work. Light is green as well.

Perhaps the MacBook Pro should be evaluated; checked, tested

by 'authorized Apple service provider' ~ Maybe the DC-in Board

&/or battery both need inspected; could need a replacement.

Among last steps suggested in most Support articles, it's said:

"If your Mac still doesn't turn on, please contact Apple Support."

Try an SMC Reset while using the charger with the battery disconnected. Observe the Magsafe LED to see if it goes out momentarily before going green again.

Unplug the power adapter from the electrical outlet for a minute to reset the charger's internal circuitry.

Make sure all external devices are disconnected from the laptop.

After that I would begin disconnecting various items from the Logic Board in case one of them is causing problems. I would disconnect the keyboard and if necessary use the power on pads on the Logic Board to attempt to power on the laptop. After that I would disconnect the hard drive cable, optical drive cable, Camera, WiFi flex cable, and the built-in Display and even the memory modules. Memory is not needed for a power on test since you will hear three beeps if the laptop powers on without memory installed. There is a slim chance the DC-In Board is bad although I've rarely seen them go bad on this model and requires removing the Logic Board to access. If all these items are disconnected and you try another DC-In Board, then the Logic Board is most likely bad.

Edit: If this is the 13" model, then it is still supported by Apple so an official Apple repair is possible if you feel it is worth spending money on it.

Edit2: I forgot to include the links for the SMC Reset and for the Power On Pads:

How to reset the SMC of your Mac - Apple Support

https://beetstech.com/blog/where-are-macbook-pro-unibody-power-on-pads-located