I’m confused about your reference to email…
CityStrides does not use email-based logins - it only connects to your MapMyFitness/Runkeeper/Strava account, which you need to already be logged into.
If you go to OneASICS | One Account. OneASICS. are you able to log in directly to Runkeeper’s site?
After you accomplish that, then you can try logging into CityStrides from CityStrides