the line to which you referring looks to define WARNING_DISPLAY_PRICE_WITH_TAX, based on values for DISPLAY_PRICE_WITH_TAX_ADMIN and DISPLAY_PRICE_WITH_TAX.
the vars in question look to be part of a base ZC install and should be in the configuration table of the zen-cart database. if they are not there, then something failed in the install of the base zen-cart.
without digging too deep, i would guess there are debug logs for the zc install and something failed due to the database perhaps being in strict mode. but alas, i am just guessing.
best.
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