As stated, the Pixelmon mod only runs on Minecraft 1.12.2. Due of this, players need to download Minecraft Forge 1.12.2 instead of the most recent version of Minecraft Forge.

In Minecraft Java Edition, players can press the F3 button on their keyboard to bring up a debug screen. Your login session has expired. Please logout and login again.

Download the "Installer.jar" file (this is an executable java archive). Double click to run it. Left click on the menu button to see all 4 options. Be sure to view the readme first. Click "Install Mod ...