Receiving this Error basically means you are trying to connect to a Modded Server and the solutions are as follows
Download the Mods directly from your server files.
a. Go to your Panel
b. Click Files
c. Find Mods Folder
d. Download them all to your Mods Folder
Do not download modpack from curseforge if you use the modpack installer. You should only use Modpacks from CurseForge if you don't use the modpack installer to avoid having different mod versions.
Re-install Mods on your Client
There might be a corrupted file on your client.
All done! Mostly these two solutions work but if you need more help, feel free to contact support on discord
Updated on: 30 / 07 / 2021