Craft 3: Can't backup database ("mysqldump: command not found")
Craft 3 did away with the PHP-based database backup solution that Craft 2 utilized (for many reasons).

Now it requires that the mysqldump executable (if you're using MySQL) or pg_dump executable (if you're using PostgreSQL) is available to PHP in order to do backups.  This is usually just a matter of including the path to that executing in your PATH environment variable.

There is also the backupCommand config setting that you can use to override Craft's default backup implementation if you have some other backup solution you'd like to use.

Same for restoreCommand.

Related issue here: https://github.com/craftcms/cms/issues/1369