I have created an internal release of an Android app on the Google Play console. When I copy the opt-in link and paste it into the browser, and then click the link to install the app, I get this Not Found page.
According to the Play Console, the Internal Testing Track is not active, but it's been 5+ days.
Here's what's not the problem:
The answer is that 48 hours may be considered typical, but my internal test was not ready until a full week after its upload to the Play Console. During that week, I put in a ticket with Google developer support, but never got a response. My recommendation is to wait for a week, and then put in a support ticket if it's still not up by then, and hope they respond.