Cause
Function keys are disabled by default.
Resolution
Inside the Citrix Workspace app, go to the Settings tab. Tap Keyboard Options. Then, tap the switches next to the function keys you wish to enable.
1) Tap the Settings icon in the lower right-hand corner.
2) Tap Keyboard Options.
3) Tap the switches next to the function keys that you wish to enable.
OUTCOME
The specified function keys will no longer be missing.