Cut a piece of self-adhesive mesh tape long enough to cover the crack plus 1 inch on each end. Stick it directly over the groove. Do not use paper tape here; paper tape will bubble over a vertical seam.
Using your 6-inch knife, spread joint compound over the tape. Press hard enough to force mud through the mesh holes. Scrape off excess. Let dry 24 hours. vertical crack in drywall
Wipe with a damp cloth. Dry completely.