Row 22: REP Row 18
Row 26: Rep Row 18
The reason this needs to be done is because see how the end of the row preceding it says CH1 turn? That means the next row is going to be pretty short on height.
The only row it will line up with properly between 17-21 to maintain the pretty pattern in the photo is 18. If 19 is done, it will work but then you lose those pretty vertical sections with the sort of V shaped motifs and instead just get boring old single crochets.
The "REP Rows 17 -28 until desired length” is correct.
Notice how the last two instructions on Row 28 are CH 3, turn? That means the next row you crochet will have some height to it - ch 1 is 1/3 the height of a ch 3. The only row that clings with this instruction is Row 17.