Problems to download Fitness on iPad Pro 11“ iPad OS 14.3.

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Devices you can use with Apple Fitness+ - Apple Support

You need a compatible iPhone paired to an Apple Watch series 3, linked to a compatible iPad to download this app.

You cannot simply install this app with only iPad alone.

I had an online chat with 2nd level support earlier today and they told me that Apple Engineering was aware of the problem. They also said that it might take an iOS update.

I just checked this and it is working now.