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.
Note that in Test Mode, you will not be able to select your scenario. The system is designed to give you a random scenario each time, so you are prepared for any situation.
Please let us know if you have any other questions or need additional assistance.