The core means which emulator are you using for the running the game. You need to use the exact version of it and also the exact ROM on both systems. Since you're running a Megadrive rom, the core should be either lr-picodrive (I think this is the default) or lr-genesis-plus-gx (see the Megadrive docs).
Thank you for your response, sorry for not providing all info. Just from your post I was able to figure it out and have Netplay working / connecting now! However, it seems as though the controllers do not work in a lot of the games while on Netplay. I have tried using different emulators to no avail. An example is Street Fighter II for Genesis, Player 1 controller works fine, but player 2 controller doesn't work at all. Tried a keyboard as well, both which work fine not in Netplay.
My system is a CanaKit - https://amzn.to/2Taj5jf
PI 3 B+ w/ 2.5A power supply
RetroPie V 4.4.4 (skyscraper-pr6-v3)
Built from premade SD image on RetroPie website
I have a USB keyboard, mouse and controller hooked up to it
Controller is working great outside of Netplay, it's an "EasySMX 2.4G Wireless Controller" and also a Geeekpi SNES USB controller
Emulator is lr-genesis-plus-gx
"... I immediately clicked "verify only" and got the error "Verification failed at sector: 8192".
Then I tried the following:
READ the contents of the SD card back to a file TEST.IMG.
WRITE the file TEST.IMG back to the SD card.
Run "verify only".
This time, the verify succeeded without any errors."
I performed the same steps and got the same outcome.
It took 3-4x as long this time to read,write, verify the image.
Some of the NES Roms now play, still working on it
Thanks again for everyones help :)
@meleu thanks friend but dont work, i try to create manually in winscp the file emulators.cfg and configurated and works but need to edit manually the file outside , if i try to modify in runcomand inside raspberry dont work and show errors again.