There are several reasons for RTXI to not open.
If you run
rtxi in the terminal and get this output, RTXI is already running:
The error happens because only one instance of RTXI can be running on the system. Check who is running it by entering:
If the command shows someone else's username, that means they have RTXI open. Bug them about closing it or close it yourself by running
$ sudo pkill rtxi. Note that workspaces, open files, etc. will not be saved when force-closing RTXI.
If it's your username, RTXI may not have exited cleanly from sometime earlier. You can run
$ sudo pkill rtxi for this case, too.
Now, you should be able to run RTXI.
Check the kernel you are using. Run this in the terminal:
You will get one line of output that tells you what kernel you are using. If it has the word "xenomai" in it, then it's a real-time kernel. If not, then it isn't.
If it's not a real-time kernel, you'll have to reboot your computer and pick a real-time kernel in the GRUB boot menu that pops up. If you don't see it, enter the "Advanced options..." submenu and look there.
Also, see this page about setting a default kernel.