Please read https://tt-rss.org/wiki/InstallationNotes.

Once you have configured TT-RSS, tweak /etc/conf.d/ttrssd to your
needs if you have not already done so. If ACLs are unavailable on the
filesystem you have just installed to then you will need to set
TTRSSD_USER to your PHP user. When everything is ready, (re)start the
update daemon like so:

  /etc/init.d/ttrssd restart

This will periodically update your feeds in the background. Add the
daemon to your default runlevel to start it on every boot:

  rc-update add ttrssd default