Why the server doesn't start?

Your server can't start for following reasons: 

- The modpack is too recent and requires java 16. (Enable it from the "Settings" menu). 

- The installed mod is not compatible with server side so you need to remove it. (For e.g. "CustomMainMenu").

- The server version is empty, it happens when you upload your own custom jar or restore a backup. So use the "SERVER VERSION" selector from the "Settings" menu to get the right one. 

- The disk space is full because of some automatic backup files caused by mod, plugin or console logs. Remove logs folder, backups folder with Filezilla and your FTP access.