The chart A and B has 8 rows of 8 stitches. The pattern says
R1: work chart A, work chart B, k20, work chart B, work chart A.
R2: work next row of chart A, etc, etc.
Then (here is where I don't know)
Repeat these 2 rows repeating chart A & B until you have 17".
Does this mean repeat only chart row 1 & 2 to 17" or does it mean work the chart through ( all 8 rows) then repeat the whole chart until 17"? I feel like it should be all 8 rows.
Know enough to get into trouble.
It likely means repeat the spacing of rows 1 and 2 (the 20 sts between chart repeats) but to continue with the succeeding chart rows, 1-8. When you finish row 8 of the charts, start over again with row 1 and continue like this until you reach the desired length. It may help to look at the picture of the finished item to make sure that that makes sense.