Greetings katsoumi,
The following article says we should begin with a couple steps - How to use Apple Fitness+
To get started
• Make sure that you have a compatible Apple Watch or iPhone updated to the latest software.
• If you haven't already, subscribe to Apple Fitness+.
If those check out, next, we’d like to have you be sure to restart all devices involved and test again.
Should the issue persist, something that would be helpful to know, as we work to isolate -
Which device were you attempting to view the videos on?
Thanks.