The queue is too large to be able to show / check for individual activities on-page. The message you are receiving when attempting to sync the activity is correct, though.
There are ~11k supporter activities in the queue.
I don’t know if it would help for you to message Strava about increasing the API limit for CityStrides
https://support.strava.com
In the meantime, I’m working on the Garmin integration. I don’t know what percentage of CityStrides users are only using Strava because there’s no current Garmin support, so I don’t know how much this will actually alleviate the problem, but it’s a move I have to make either way.
I’m entirely overwhelmed with the activity in this forum, so please don’t take offense if I don’t reply further to this thread. 