Your best bet is to read the log to see what mod(s) are causing the issue. You also need to make sure that you’re using the right version/modloader that the mods need.
I would assume so, but have you actually unzipped the zip file?
Other things to look out for/check:
Have you enabled the correct modload for your modpack? If yes make sure that the modloader version is the exact same as the modpack requires.
Have you added the mods via the file manager in the GUI or on the acutal server? If you added them on the filesystem of the server check if the permissions are correct.
Are you using the right Java version for the MC version?
Then the server failed to run or doesn’t let me in
This is very vague, if you post the startup log I could tell you more.