I downgraded my app this month to free plan due to financial issues expecting that I could still work on it but I see now that I am unable to view my app in preview mode. Is this really the case? Seems unbelieveable
Nope, you should still be able to preview your app.
Any error message?
The app has zero issues. When I click the preview button it takes me to this link:
Ahh - I believe you cannot have a custom domain on the free plan. Try removing that from your App settings and then preview should work.
So it seems you had a domain previously connected. And there is no option to connect domains on free plan.