When I tried to synchronize my site.db to github.com, something wrong happened. There is an error on nginx 502 Bad Gateway.

When checking the error log on my server at:
cat /var/log/flaskblog/flaskblog.err.log
I found the error.

1
sqlite3.OperationalError: attempt to write a readonly database

To change the authority of writing in this database, I used:
chmod a+rw ./site.db
to give all users the authority to read and wirte this database file.

This solves the problem.