Do you mean that the cast-on edge has a gap or ‘jog’ between the first and last stitch? If so, there are a few solutions:
Knit the first stitch extra-tight, possibly holding the cast-on tail together with the working wool and knit normally
Or cast on an extra stitch, pass it from the last needle (last cast-on stitch) to the first needle, beside the first stitch, and make your first stitch a k2tog, then knit the first few super-tight, possibly knitting the tail as above
Or the method I love, which gives me a flawless join, where you pass the last stitch over to the ‘first’ needle and put it on the needle, next to the first stitch, then pass the first stitch over, and put it where the last stitch used to be, the two stitches are switching places with one of them going through the other (doesn’t matter which, I think). A crochet hook is a big help here. Then I knit extra tight holding the tail tightly together with the working yarn for 6-10 stitches. But I am a fussy perfectionist.
I know it’s too late for this item. For this one, you can use the tail to sew up/close the join when you weave it in. And until it closes, knit the first few stitches of each round - on top of the join - extra-tightly.