Why can't I Ctrl + Alt + Del on the On Screen Keyboard?

If UAC is disabled on the target computer then passing a fake Secure Attention Sequence (SAS) keystroke (Ctrl+Alt+Delete) to the system (ie: from an OSK or a remote-control program like VNC) will fail.

If you need lowered UAC interference, then you can turn it down to one notch above Off and that should allow the SAS to be passed.

Similar question is here: Send Ctrl+Alt+Del over VNC with Windows Vista and UAC off?

Ctrl-Alt-Del is a secure keyboard combination that cannot be passed through the On-Screen Keyboard (AFAIK).

You can read more here at Wikipedia.

Check the tablet quick start guide or operating manual for a Windows Security button. This acts as an equivalent for the Ctrl-Alt-Del key combination.

The tablet does not come with a keyboard, right?

You can try the onscreen keyboard.

But you need to use "AltGr" instead of "Alt".

Ctrl + AltGr + Del

AltGr is on the other side of Space button.