as stated, you have a Cup game this week. After you are booted from the Cup, you can challenge teams.
However you can NOT issue challenges until late Friday (Boston Time) for the following Thursday.
How is this done? At the bottom of each team's overview page, there will be either a statement saying that the team "cannot be challenged to a scrimmage due to tournament commitments", or 'this team has already scheduled a scrimmage'.
If you are not in the Cup, and another team is also out of the cup, then you will see a drop down box, and as stated, it will be at the bottom of the another team's overview page.