Jar files require a manifest file, which must be located inside the file under /META-INF/MANIFEST.MF
The manifest file needs to contain specific information so the .jar can be run. Many items on that link are optional, such as the methods for signing the jar, but a minimal jar would probably be:
I've tried editing the Manifest file several times in different ways but it still gives me the same error. I extracted the contents of the .jar, edited the manifest file and used 7zip to open the .jar again to overwrite the manifest with the one with the changes I made. I even used the exact same text you gave me. I'm still getting the same error.