Selling Price (required) is what the customer pays. Regular Price is optional and meant for showing a discount (its placeholder says "insert the regular value if the item is with discount"): when set, the website can present the regular price as the crossed-out "was" price next to the selling price.

Because it represents the higher, pre-discount amount, Sprwt refuses to save when a Regular Price is entered that is less than or equal to the Selling Price — you get "The selling price should be less than the regular price". Leave Regular Price empty (or 0) if there is no discount to show. Both are shown as columns in the packs list.