Okay I changed the server type to Forge. I downloaded the Pixelmon mod pack from Forge. Now I’m wait for the jar files to upload into the plugins folder. Correct?
No, modded servers use a mods folder
You want to download the Server modpack (at the bottom of the page under Additional Files), otherwise there may be some incompatibilities from client-only mods.
Just upload all the files from the server pack into the main directory of the Minecraft server