When configuring the package manager, I get this warning:
An error has been detected while trying to use the specified Debian archive mirror
If I type Ctrl+Alt+F4, then I get this warning:
mirror does not support the specified release (stretch)
Check your internet connection. In my case it was I did not pointed out :port when been writing proxy server address.
I recently had this problem. I downloaded a testing/buster iso on the same day it was released. Needless to say, the mirrors hadn't been updated yet. My cd image was dated March 18, 2019 but all the mirrors still had a February version.