This means that the full Forge installation is not present in the correct location of the server files.
When uploading a custom Forge installation please make sure that the Forge jar that the server runs off, the Vanilla server jar of the same MC version and the full libraries folder are ALL present in the servers jar folder. If the Vanilla jar or any of the contents of the libraries folder are not present then the server will be unable to load Forge.
You can get all of the relevant files needed for this when running the Forge installer locally and selecting the Server installation option.
If this is using any of the 1-click install options please log into the server files and delete the jar folder, then restart the server - this will force reinstall that option. If this does not help please submit a support ticket and we will resolve this ASAP.
NOTE - If this is using one of the plain Forge options on the jar list please be aware that force reinstalling the option, or switching to a different plain Forge option, will wipe the mods folder on server load!