цветастик, Client forge version is different than server. Client has missing mods the server requires. Client has mods that’s required on server. Client has different mod versions than server mods. Mismatch data (configs, datapacks, etc) between client and server. You (the client) and server should both have the same mod loader & version, mods & versions, configs, etc. Client could have additional mods as long as they’re client-side only. If friends using CurseForge, you should use CurseForge, though you don’t need Curseforge launcher. There’s FTBApp, PolyMC, Prism, ATLauncher, GDLauncher, and a couple others but these launchers can download mods & modpacks from Curseforge.