In this guide
Missing Forge or Fabric Mod Loader
If you're getting an error message when joining your server, first check to make sure you have a mod loader installed. Mods will require Forge, NeoForge, Fabric, or Quilt to work. This will differ based on the mods you're using. Make sure you're running a mod loader and that it is the correct one for your mods.
Fabric mods can be loaded on Quilt or Fabric Forge mods can be loaded or Forge or NeoForge Quilt mods can only be loaded on Quilt NeoForge mods can only be loaded on NeoForge
Missing Mods
Make sure make the server and your client have the same mods and configs. Having mods on the server that are not on the client, or mods that are on the client that are not on the server will stop you from joining.
Time Out Issues
If the mods and configs match, and you are running a mod loader, you might be getting a connection or timeout error. A simple fix for this is to install this mod on the client and server: [Connectivity Download] (https://www.curseforge.com/minecraft/mc-mods/connectivity)