There should be no issues applying TotalBoat Tabletop Epoxy over a cured urethane or varnish, but there are some very important things to consider.
The urethane or varnish needs to be completely cured, ideally for at least 72 hours under normal temperature and humidity conditions, and it must not emit any solvent odor. It is also strongly recommended to pour a thinner layer of TotalBoat Tabletop Epoxy rather than a thick one. If the epoxy's cure generates enough heat, it can weaken or delaminate the underlying film of urethane or varnish, leaving a bubble or a potential milky looking, or cloudy spot underneath the previous product.