x17 R2, keyboard key not working

Connect an external USB keyboard and try that for a while. If the issue also occurs with the external keyboard, then the issue is something in Windows. If the issue does not occur with the external keyboard, then it's likely a hardware issue with the onboard keyboard and it may need to be replaced.

Here are some other things to try:

• Try a power reset (unplug AC adapter, disconnect battery, hold down power button for 30 seconds to discharge flea power. Then connect the AC adapter and try it).
• Go into Device Manager and delete the keyboard driver. Windows will install the latest.
• Use the keyboard troubleshooter. Troubleshoot settings | Additional troubleshooters | Keyboard
• Try in Safe Mode to see if the issue occurs there.
• Try the Windows on-screen keyboard. If on-screen keyboard also exhibits the issue, it could be due to corrupted system files. Run SFC (System File Checker) scan on your PC to repair those files.
• Try in Linux (by booting from a flash drive with Live USB install of Ubuntu, Xubuntu or other)

Dell Keyboard Usage and Troubleshooting Guide 

If those procedures point to a hardware issue with the keyboard, contact Dell Support while you're still within warranty.