how to install watchOS 7 profile

That is a developer website error, so all I can suggest (since I am getting it) is to keep trying until Apple fix the error on the website.

But it turns out there is a bug in the link, from a response to a similar question:

This URL works for me (browse to on iPhone):

https://developer.apple.com/services-account/download?path=/WWDC_2020/watchOS_7_beta_Configuration_Profile/watchOS_7_Beta_Profilemobileconfig.mobileconfig

Yes, it appears that Apple has revised the link to download the watchOS 7 beta profile. Going through the developer web site now works. Thanks.

(yes, browsing it from the iPhone)