Fitness+ down?

Yes, the service is down.

https://www.apple.com/support/systemstatus/

M1 Mac mini, macOS 11.2.3; Watch, series 5 GPS/cellular, watchOS 7.3.2; iPad Pro 2nd Gen, iPadOS 14.4.1; iPad Air 2, iPadOS 14.4.1; iPhone 12 Pro, iOS 14.4.1; TV 4th Gen, tvOS 14.;  HomePods, audioOS 14.4.1 and Airport Express w/ Airplay 2

Same issue here - I’ve been using it for a while now though, first time I’ve had an issue. Disappointing as can’t find anywhere on the app to report an issue.