OK, thank you.
It turns out the column ls_id is improperly defined in ls_dr8.tractor_n. We are working on a fix for this, but in the meantime, you should be able to perform the join as so:
SELECT f.ls_id, f.ls_ccd_id, t.ra, t.dec
FROM ls_dr8.tractor as t
JOIN ls_dr8.forced as f ON (f.ls_id = ls_dr8.ls_id(t.release, t.brickid, t.objid))
WHERE t.dec > 40