This tutorial will help you properly install a Minecraft modpack.
First, download your modpack server files from wherever you got the modpack. In this example, we are going to use Tekkit Legends.
The files in red can be deleted as they are not needed.
Rename the TekkitLegends.jar file or any jar file in the root directory excluding the Minecraft server jar to server.jar or anything that is convenient.
Upload the files to the server root directory via SFTP.
Go to Configuration > Startup Parameters on the panel.
Rename the server jar file parameter to the name of the modpack jar file.
Special blocks not serversided making the client not in sync with the server creating air within a couple seconds of placing a block
Certain scripts/tools not working
If you get one of these issues then the server jar file is not forge and is instead vanilla Minecraft.
If your server crashes, you will see an error code and a message along with the status of out of ram. If you see that the status is false, read the logs before the error message. There is something wrong with one of your mods or your world file or it could be something completely different. If it is true, then you will need to remove some mods or purchase extra ram because your server was out of ram.
If you have any issues, please feel free to open a support ticket.