MazziBellas Please clear the EA App's cache:
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
Turn on automatic game updates, under App Settings > Downloads, then repair the game (Sims 3 > Manage > Repair). Launch it once and see whether the download starts on its own.
If that doesn't help, please try the workaround in the accepted solution of this thread:
New Sims 3 expansion: says in-game content, can't install | EA Forums - 8323544
You'll of course do this for the Sims 4 folder, not Sims 3. City Living is EP03.