Google Play Alpha: App not available for this account

I released my app on Google Play Alpha. I added testers through lists of email addresses by selecting "Closed Alpha Testing" under "Choose a testing method". So to be clear, this is not a duplicate of this question, since I'm not using Google groups.

However, when I then open the Opt-in URL (https://play.google.com/apps/testing/com.my.app), I get the message "App not available for this account":

I'm 100% sure I'm signed in to the right Google account, and everyone else I invited also gets this error. I also did not receive any email regarding the invite, like some people suggested should happen when I googled it.

Currently, it's been about 2 hours since I've published the app to the alpha track, and nothing has changed.

How can I test the app on the closed alpha track?

It took a while to activate. It seems now (a day later), the link does work. Though this delay wasn't clear (to me at least) from the Play Console interface.

Note: I also never received an email about the activation.