You can't use Java 15 with Forge right now, Forge needs a component of Java that was retired in 15.
Use Java 8 HotSpot (64 Bit) for full compatibility (Forge should work on anything between 8-14 but 8 is the target because it's what Mojang ships with the client).
To my knowledge Pixelmon is still on 1.12.2, and is a mod that has to be on both sides (client and server), if you're trying to join 1.16.3 servers with 1.12.2 that's not going to work.