Missing Required Core Mod or Plugin:

This means that one of your Mods or plugins requires another mod or plugin and it is not installed. 

Modpack solution:
- Reupload the modpack to the server.
- Look at the mod dependencies page to see what mods the mod needs and check the console to see if it specifies what mod it needs.

Plugin solution:
- Look at the plugin's Spigot/Bukkit page and check if it lists any 'dependencies', otherwise known as needed plugins for it to run.
Common plugin dependencies include:
- Essentials
- Vault
- Groupmanager/PermissionsEX
- ProtocolLib