https://www.gamespot.com/articles/destiny-2-nightfall-bug-discovered-last-chance-to-/1100-6453131/

Update 2: Bungie has announced that one of the modifiers for Destiny 2's current Nightfall, Prism, is not working properly; specifically, the element "is not persisting on screen as is intended." Additionally, it's found a "more severe issue" with the Momentum modifier that was meant for use with the next Nightfall. As a result, Bungie plans to bring back the Prism for a second week to stop the Momentum issue--whatever it is--from impacting players. It noted that these bugs will be fixed in a future hotfix, but there's no word on how soon that update might arrive.

Update: If you're hoping to try out Destiny 2's first Nightfall Strike, you're running out of time to do so. The next Nightfall will be introduced tomorrow, September 12, as part of the weekly reset. If you're sufficiently equipped to take on the challenge--which requires hitting 240 Power level, prompting a notification from Zavala--you can jump into the activity from the Director. Just keep in mind you'll likely need a group of three to take it on. The original story follows.