I had a go and got this working. Here’s what I did.
Mineos UI
Profiles - Select “Forge” from the drop-down and then selected “Download” for 1.8.9-Recommended (build 1722)
Once that was downloaded, I went to “Create New Server” and called it pixelmon.
Went to the server status page, selected 1.8.9-Recommended from the change profile, then clicked Copy profile to live server files
Selected 1722-installer.jar from the Change runnable jar, then started the server
The server stops and now has 1722-universal.jar, so I selected that and started the server.
The server stopped as I had to accept the EULA
Started it again - and it was up
Stopped the server
From a putty session to the mineos server I went into my games folder, then the server pixelmon folder, then the mods folder. I then used wget to grab the file (http://download.nodecdn.net/containers/pixelmon/core/Pixelmon-1.8.9-4.2.7-universal.jar)
Started the server and success.
Can you go to the mods folder and check who owns the file? It should be “mc”, not root - in case you used root to copy it there…