Java could not find the main class it was told to run
What it looks like in your log
Error: Could not find or load main class net.minecraft.server.Main
What causes it
The start command names a class that is not inside the jar it points at, so nothing starts. Either the jar is not what it should be (a download page saved as .jar, a zip, a loader jar for another version, a partial download) or the command was copied from a guide for a different server type.
How to fix it
Check the jar the start command runs is the real server jar (tens of megabytes, not kilobytes) and re-download it if in doubt. If you use a run script from the Forge or NeoForge installer, run the installer again for this exact version so it rewrites the arguments file.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log