Hi and welcome to KH!
Sometimes the pattern wants you to decrease on the purl side of stockinette and that could be the case here or it may be just a matter of being off one row. At any rate, if it's easier to do the decreases on the knit side, then go ahead and put them on the knit side, (RS). I prefer that myself. Since it occurs after 20 rows plain, it won't matter if the decreases start up again on row 19 or row 21.
Enjoy finishing up your sweater!