It looks from the knit side like that stitch extends over more than one row. On the purl side you can see the yarn skipping by the stitch. I think you just missed working the stitch. You could work to the column that the st is in and then drop the st in the current row off the needle and let it ladder down to the mistake, fix the error and then pick up the ladder to the current row.
Or you could leave it. If you decide to leave it, practice this kind of mistake on a swatch, so that you’re more comfortable repairing it the next time it happens.