Comments - Can't install on Debian 9. "E: Unable to locate package mariadb-backup"
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This means that package can't be found within your repositories, so the question is which repo are you using?
Honestly, I was having this problem at a third rate hosting company called Hostwinds. I moved over to DigitalOcean and all worked perfectly. My counsel to anybody in a similar position — don't use Hostwinds.