@kenriley001 With beta 54, the behavior is slightly different. Regardless of adblock setting, the "can't be reached" message frequently appears on the first attempt at download. Closing the tab or closing Opera, then retrying the download is successful.
It appears there is something peculiar about apkmirror. I have never been able to whitelist this site with Opera, and a Google search reveals this is a long-standing problem with some ad blockers.