Why is TT Premier/Mac (up-to-date) calling cost-basis overrides I had to enter on IRS Form 8949 "errors" I need to fix by removing them (which will cost a LOT of $) ?

Box E is checked: the broker did NOT report the cost-basis (no idea why not), but did supply a "Supplemental Information" PDF that lists those values.  Since these "cost-basis" values — what I paid for the stocks — are almost the same as the sale prices, the tax implications are very costly.