Hi and welcome to Knitting Help. When knitting flat, you alternate rows of knits and purls to get stockinette stitch. When knitting in the round, it's all knits, all the time. My guess is you tried to do stockinette with alternating knit and purl rounds as for knitting flat and you need to knit every round.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper