You need to login to the sandbox user on the phone. Go to the settings and open the account details. It tells you that you need to verify your account. Click on send verify email on the mobile device which will send a different email with a code inside the email. After you received the code you need to enter it on your device. This is how I was able to bypass the blank activation link.

The issue is not with the sandbox being broken. Issue is being that the verify account email that is being sent to the sandbox user, having a broken link inside of it. When I click verify it takes me to a blank page. Console shows lots of 404 errors. Meaning you can't verify the sandbox user and you won't be able to enable apple pay etc for that sandbox user. Steps to reproduce: Login to the appstore connect Go to the users and access section Go to the sandbox selection in the sidemenu 4.Add a new sandbox tester 5. Make sure you add an email you have access to 6.Apple sends an email to verify the account 7. Click on the Verify Link. The link points to a blank page and the account is not being verified.

I ran into a similar issue and had a month long discussion with apple people. Seems like this designed like this and you should switch to a different url.

Then you cannot do anything. Seems like you need to be able to access the email as verification link or code is sent there. Depending what method you use.