Hm… This part looks a little strange… Does fly checks list -a db-app-name also show that MISSING parenthetical warning?
Here’s a normal check, for comparison:
[✓] checkDisk: 866.33 MB (89.0%) free space on /data/
(That was on a 1GB volume.)
Aside: If this is a single-Machine database then it might be prudent to review backup strategies, etc., at this point, 
.