There are three sets of keyboard shortcuts:
Default - these are available to everyone.
SDLX - these shortcuts will only work if you have selected the SDLX user profile.
SDL Trados - these shortcuts will only work if you have selected the SDL Trados user profile.
Depending on what profile is selected, different keyboard shortcuts can be used. To check which user profile is currently active, select File > Setup > Manage User Profiles. The Manage User Profiles dialog is displayed. From here you can switch profiles, export your current profile to a file or reset your profile settings.