Please make sure you haven't reached the maximum amount of Multiplayer races.
If you did, please delete some races by following the steps below:

1. Open the Multiplayer menu,
2. Check your list of Multiplayer Matches. You can use the buttons "Most Played", "Most Recent", and "Groups Only" to filter the list of matches.
3. Delete some of the matches by clicking on the "X" button in the corner of your opponent's picture.

If you haven't reached the maximum number of races, please make sure that you have a stable and strong internet connection. Furthermore, check your device's settings to make sure that Date and Time are correct and are Automatically Set.