Hi… It looks like you might perhaps have misread this piece of the tutorial (understandable!). The one there is instead sqlite:////data/db.sqlite3, with four slashes immediately after the colon and then the (crucial) data/ step in the path.
(That latter should correspond to destination under [mounts] in your fly.toml file.)
In general, the Fly.io platform doesn’t synch volumes. You’re expected to instead either start over from scratch in production (typical) or, more rarely, manually upload your initial database via SFTP.
Aside: I understand what that other tutorial is saying about the complexity of multiple machines, but having just a single one really is not recommended on Fly, 
. In particular, this database could be lost in its entirety, unless you watch things very assiduously…