There are 3 different simulation modes to choose from: Learn, Practice and Test.
If you already have a simulation scenario selection window open, the other simulation mode options will be greyed out. Is that what you see?
If so, simply click on the X at the top right of the scenario selection window to close it (where the cursor "hand" is pointing in the screenshot below):
Now you'll be able to select one of the other simulation modes.
Please let us know if you have any other questions or need additional assistance.