If you made your Minecraft account using a Microsoft account, you cannot use Pixelmoncraft Launcher, Technic Launcher, AT Launcher, etc. You have to use the default Minecraft Launcher, with Forge for 1.12.2 installed.
Keep in mind the account still needs to have the Java edition on it, Bedrock or Windows 10 version of minecraft will not work!
Link for Forge 1.12.2
After that is installed, go to %appdata%/.minecraft for Windows, or ~/Library/Application Support/minecraft for Mac, in your My Computer files, and create a folder called mods. Make sure it's all lower-case.
Put both of these mods into the mods folder you created.
Now you should be able to open your Minecraft Launcher and run the Forge version you installed, and you'll be able to play Pixelmon on our server.