Try setting it to MODS
Disconnecting Player: This server has mods that require FML/Forge to be installed on the client: [Pixelmon, Reborn Core]
Really unbelievable brother, you helped me a lot by asking me to put it in waterfall, I managed to discover the error and even fix it. There were 2 modpacks in the mods folder that had bugs, so I removed them and recreated server 0 with proxy lobby and the pixelmon server with pixelmon mod and sponge then I applied the plugins little by little and it worked and at the end of all this I discovered that someone on my team I had added a journey map and among other mods that were leaving the pixelmon server, buggy friend, I cried after that, I spent about 4 days trying to solve it, only to see in the end that there were 2 mods. I don’t know if I’m happy with happiness now or if I cry with sadness knowing that my friend is stupid.
Glad you managed to solve it 