Perhaps someone will explain the subtle nuances of the phrase at the end of paragraph G which says:
Within 21 days after the effective date of this AD, inspect the rigid and full flexible couplings for correct assembly, including replacement of the o-rings with new o-rings, confirmation that the proper retainer rings are installed in the full flexible coupling, a general visual inspection for damage of the blade seals, and all applicable corrective actions. Do all applicable corrective actions before further flight.
OK, reasonable question, your confusion is understandable.
Think of it as a Catch-22. An operator can wait 21 days, but no longer, before performing that particular inspection and, if necessary, applying any corrective actions. No check within 21 days means the aircraft is then grounded.
But if the check is done sooner, and a fault is found, the operator can't then fly the aircraft for the remainder of the 21 days before carrying out rectification - that must be done before the next flight.