"Application not installed" for any "Actions" application for which you want to link accounts

I'm working on an Assistant app that requires linking an account, and I've gotten to the point where if I say "talk to [my app name]", I'll return a card with the text "Link [my app name] to Google." Clicking on it, however, gives me a (incorrectly formatted) toast with the text " Application not installed ".

I thought I did something wrong, so I tested other apps that require linking accounts. Alarm.com, Mail Buddy, they all behave the same. How should the user "install" these applications?