What is the worth of a 1928 red seal two dollar bill missing the phrase In God We Trust?

NO bills printed before 1957 had that motto on them so your bill is NOT an error. The motto was only added in response to the anti-communist hysteria sweeping the country at that time (see the current flap over dollars with the motto inscribed on their edges...)

You'll need to post a new question with the bill's date, seal color, and whether there is a tiny letter next to the date. Without those details it's not possible to provide a value.