I can't send a Ctrl-Alt-Delete combination to my Windows computer

There is a default system policy in Windows 7 and above that prevents system services from sending Ctrl-Alt-Del (the Secure Attention Sequence, "SAS").

Note that only RealVNC Server running in Service Mode (this is the default mode on Windows) may process this key sequence sent from a connected RealVNC Viewer.

How do I resolve this?

There are two ways to solve this, either by setting the group policy to allow services to simulate SAS, or by overriding the group policy in place. Either: 

1.  Set SimulateSAS to 2 in the Expert tab of the RealVNC Server Options dialog which will override the domain policy for RealVNC Server, or;
   Since the server runs as a local system account with administrator privileges, the server is able to momentarily change the group policy to allow it to inject the SAS whenever it is sent from a connected RealVNC Viewer.
2. Check that the "Disable or enable software Secure Attention Sequence" policy is set to allow "Services" on the RealVNC Server PC.