It does not matter whether that first stitch is a knit or purl, it will not impact the whole of the project. It will hardly been seen if left open like a shawl, it will be hidden in the seam when you make an item out of several panels.
If you feel is it important, you can make one (or a few) extra cast on stitches and take out that first (or those extra) one(s) leaving all the stitches knit or purl as you like. You can undo the extra cast on stitches easiest if you left them drop from the needle when turning around at the end of the row, and cutting through the loop of the slipknot if they do not fall away naturally.
The way I learned to cast on there was no way to make knit or purl, it was just cast on and it worked. I have later learned a knitting way to cast on and in those you can make a difference but it mostly does not matter.