If the volume of a prism is 999 square inches what is the volume of a pyramid with the same base?

If the VOLUME is 999 SQUARE inches then you have gone wrong.

SQUARE inches are a measure of AREA

VOLUME is measured in CUBIC units.

Assuming you meant The volume of a prism is 999 CUBIC inches, then the volume of a pyramid with the same height is indeterminate (cannot be given), as there is NO INDICATION of how the height of the pyramid relates to the height of the prism.

So, ASSUMING you mean the volume of a prism is 999 CUBIC inches AND the HEIGHT of the pyramid is the same as the height of the prism, then the volume of the pyramid is one third that of the prism, ie 999 cu in ÷ 3 = 333 CUBIC inches.