A few more steps...
iOS 15 Developer beta profile has started downloading.
Make sure you open BetaProfiles.com in Safari browser.
You will see the following prompt:
Tap Allow from the popup.
Open Settings app - Profile Downloaded.
Then follow the prompts to install configuration beta in Settings app and restart your device.
After your device has restarted, you can receive beta update in Settings - General - Software Update.
You only need to install the beta profile once, the next updates will appear in the Settings app - General - Software Update.