How do I protect the wall when pouring epoxy on an installed countertop?

Don't tape the wall itself. The epoxy tape is meant to cover a removable dam, not a surface that stays in place. If you tape the wall or baseboard and pour epoxy right up against it, the tape ends up stuck in the epoxy layer and removing it is very difficult. If that has already happened, carefully cut it free with a razor blade, or gently warm it with a heat gun to soften the epoxy before peeling.

You have three better options:

1. Build a temporary dam along the wall edge. Cover a 2x4 or a piece of wood trim with epoxy tape, set it against the wall, and seal the seam with silicone caulk. This keeps the epoxy from running down between the counter and the wall, and after the 3-day cure the board comes away as one piece. 2. Install the permanent trim or backsplash first and seal the seam with latex acrylic caulk (it's permanent), then pour up to the trim. 3. If trim will cover the wall edge later, you can tape the wall, pour, and leave the tape in place under the trim.

Cover anything else you want to keep epoxy off with 4 mil painter's plastic; epoxy won't stick to it.

If you dam off part of a surface to re-pour only a section, expect a seam between old and new epoxy; the only way to avoid one is to re-pour the whole area.

Dam guide: https://ultraclear.helpscoutdocs.com/article/223-temporary-dam-epoxy Epoxy tape: https://www.bestbartopepoxy.com/collections/tools-supplies/products/epoxy-edge-tape