Try these solutions in order. The first method resolves the issue for 80% of users.

"Unable to Open Bigfile.000" Rise of the Tomb Raider typically indicates that

This issue has plagued PC gamers for years, often appearing immediately after installation or following a system update. The error is essentially the game’s way of saying, "I cannot read my own vital data files." Since the .000 file is a core archive containing game assets like textures, sound, and levels, the game cannot start without it.