How to Fix a Missing USB MIDI Keyboard in LMMS on Endless OS (Flatpak Guide)

Back in 2020, I could easily connect my USB MIDI keyboard to amsynth on Ubuntu right through the USB port without a single issue. Later on, I switched over to Endless OS, which comes with LMMS as its primary digital audio workstation. Yesterday, I plugged my keyboard into my Endless OS machine, but it completely failed to show up in the LMMS dropdown menu—even though running the lsusb command confirmed that the operating system knew it was plugged in! On Endless OS, applications run inside isolated environments called Flatpaks. While this keeps your system highly secure, it creates a sandbox wall that blocks apps like LMMS from "seeing" your raw USB hardware. To make matters trickier, the operating system often keeps the internal MIDI sequencer drivers turned off by default. Thankfully, the fix is incredibly simple! Here is the exact, step-by-step solution to wake up your system drivers, break through the sandbox, and get your MIDI keyboard making music. Step 1: Force the System to Load the MIDI Driver Before LMMS can see your keyboard, we have to tell Endless OS to activate its core Linux MIDI system. We do this using the built-in Terminal. 1. Open the Terminal app on your Endless OS desktop. 2. Type or copy-paste the following command and press Enter: sudo modprobe snd_seq_midi 3. Note: If prompted, type your computer's administrator password. No characters or asterisks will show on the screen as you type—this is a normal Linux security feature. Just type it out and hit Enter. Step 2: Grant LMMS USB Hardware Permissions Now that the operating system recognizes the keyboard, we need to punch a hole through the Flatpak sandbox so LMMS has direct permission to read your physical USB ports. 1. In the same Terminal window, type or copy-paste this command and press Enter: flatpak override --user io.lmms.LMMS --device=all This command instantly updates the application permissions. You can now close the Terminal app completely. Step 3: Map the Keyboard Inside LMMS With the permissions and drivers fixed, you are ready to link the hardware to your instrument tracks. 1. Unplug your MIDI keyboard's USB cable, wait 3 seconds, and plug it back in. 2. Launch LMMS. (Pro tip: Always plug in your hardware before opening the music software so it can scan for it on startup). 3. Look at your Song Editor and click directly on the name of the instrument you want to play (such as the default TripleOscillator) to open its settings window. 4. Click on the green MIDI tab at the top of that window. 5. Click the small piano keyboard icon ⌨️ under the MIDI INPUT section so it lights up green. 6. Click the dropdown menu right next to it (which defaults to None). You will now see your keyboard listed! For devices like the Nektar SE25, select SE25 MIDI1. Press a physical key on your keyboard—you will see the virtual piano keys move, and you will hear your instrument loud and clear! Behind the Scenes: Why Popular Tools Fail If you search online for Flatpak permission issues, many Linux forums suggest installing a graphical manager tool like Flatseal. However, Flatseal alone will fail to fix this specific problem. This is because Flatseal can only toggle application permissions; it cannot load missing system drivers. Because Endless OS leaves the underlying driver (snd_seq_midi) completely asleep by default, bypassing the sandbox doesn't help until you explicitly wake up the hardware using the sudo modprobe command in Step 1. Using the terminal solves both halves of the puzzle flawlessly! Wrap Up The best part about this fix? You only have to do it once. Endless OS will remember these settings permanently. From now on, whenever you load up LMMS, your MIDI controller will be ready to compose right out of the box. Happy producing!