Development found a few forge singleplayer mods on your account that they deleted.
Singleplayer mods aren't compatible with the learntomod server anymore, and they can sometimes cause server loading issues for users, Instead, we recommend using multiplayer.
Also, another common problem to look out for, if you see this error again, is missing main functions main (lowercase) is the signal that tells our software to begin running mods, so if it's missing, it can lead to server connection problems.
For those reasons, when downloading / copying mods that aren't yours, I'd recommend choosing mods that are in the curated mods section. Then again, those problems I mentioned are easy to fix! Just delete the mod or edit it.
If that didn't solve it, please let me know the name of the mod and take a screenshot of the error.
To take screenshots in minecraft, press f2 or fn + f2 on the keyboard then look in your computer's minecraft < screenshots folder.