Apple Fitness+

You can't see the workout this way because it is copyright protected and can't be shared from the iPhone to your TV in this manner.

This is why you need one or more of the Apple devices listed in fiery's comment to use Apple Fitness+.

M1 Mac mini, macOS 11.1; Watch, series 5 GPS/cellular, watchOS 7.2; iPad Pro 2nd Gen, iPadOS 14.3; iPad Air 2, iPadOS 14.3; iPhone 12 Pro, iOS 14.3; TV 4th Gen, tvOS 14.3  HomePods, audioOS 14.3 and Airport Express w/ Airplay 2

With an Apple TV device.

Apple Fitness+ compatibility.

Apple Fitness+ requires Apple Watch Series 3 or later with watchOS 7.2 or later and one of the following Apple devices: iPhone 6s or later with iOS 14.3 or later, iPad with iPadOS 14.3or later, or Apple TV with tvOS 14.3 or later.