What is a 1999 one dollar bill without a serial number and seal of the treasury stamp missing worth?

US bills are printed in multiple passes, and sometimes a pass is accidentally missed so the parts of the image put on by that step are missing; there's also the possibility that they were printed but on the wrong side of the bill.

Your best approach would be to have the bill evaluated by a dealer or appraiser who's familiar with error currency. In any case, without seeing it, a ballpark estimate for a missing seal could range anywhere from $200 to $500 depending on the specific cause of the error.