Forge 1.9 has been in development since they forked it on github the 31.march. You asked about this the 5.th of april. You can see their commit history here: https://github.com/MinecraftForge/MinecraftForge/commits/1.9
They have not merged the 1.9 branch with their Master branch (their current main release) yet, so you can safely assume that they are not happy with it yet. Also if you take a look at the issue list of Forge you can see that there are several issues with forge 1.9: https://github.com/MinecraftForge/MinecraftForge/issues
MineOS automatically tries to keep up with releases from the different server vendors, and does so by subscribing to their lists of releases. It then lists the available releases for you to download. If this list contains errors in the download adress MineOS wil not be able to download it. This is what seems to be the case with forge. They have either changed their download URL for 1.9, or entered the download address inncorrectly. This means that MineOS does exactly what it is asked to do, but fails to give the wanted result since the supplied information is wrong.
You can however download the jar manually, and manually place it in the server folder. This does the exact same job, as the server does when you press the download button in the profile list.
This is still a different bug report than the original report in this thread.