Error: Unable to access jarfile

What it means

The server tried to start a jar file that isn't there:

Error: Unable to access jarfile server.jar

Java was told to run server.jar (or whatever name your server uses) and couldn't find it in the server's top folder.

Common causes

• The jar was deleted or renamed. Often after uploading files from another host, or tidying up the Files tab.
• It's in a subfolder. Uploading a zip and extracting it can leave everything in a folder inside the top folder, for example myserver/server.jar instead of server.jar.
• A different name. Some downloads are called paper-1.21.4-232.jar or forge-...-installer.jar, not the name the server expects.
• A modpack's server files that need their own start script and haven't been installed properly.

The fix

1. Open the Startup tab.
2. Check the server software and version are what you want, and save them.
3. Start the server. It fetches the server files it needs for that software as it starts.

If you've set a custom start command or jar name, check the name matches a file in the top folder exactly, including capitals, or switch back to the standard startup.

If you uploaded files from another host and they're in a subfolder, move everything up into the top folder with the Files tab or SFTP, so server.properties and your world folder sit at the top.

Don't reach for Reinstall to fix this: it deletes everything on the server, worlds included.

Modpacks

Don't upload a modpack's client files and expect them to start. Use the modpack options on the Startup tab, which install the pack's server files for you.