Can I use shellac on wood before applying epoxy?

No. UltraClear Bar & Table Top Epoxy cannot be applied over shellac, so do not put shellac on a surface you plan to epoxy.

If you want to add color first, use a water-based or alcohol-based wood stain. If you want a clear finish under the epoxy, it needs to be an oil-based polyurethane. Let it fully cure, sand it with 220-320 grit, clean it with acetone, and then begin with the epoxy seal coat.

None of this is required, though. The epoxy can go directly onto the raw wood: on wood or any other porous surface, you start with a brushed-on seal coat, then pour the self-leveling flood coat. Full details are in the instructions: https://cdn.shopify.com/s/files/1/0320/7168/0140/files/Epoxy-Instructions.pdf?v=1616443869