Not "push against." Reaction mass. Assuming a "pressor field" or "repulsion ray" can be actually built, one still has to use it to throw a mass in the opposite direction to produce thrust. Filling your rocket with mass, and tossing it out the back would work. Rocks. Trash. Liquids and gases. Repelling a large, nearby mass would also work, pushing your vehicle rapidly away while applying only a small delta-V to the much larger mass.

I used an asteroid as an example because I don't think this technique can generate anywhere near enough force to lift a space vehicle away from a planet.