In the final battle scene of Harry Potter & The Cursed Child,
knowing he was going to face Delphi, why didn't Harry get the Elder Wand?
At the point that Harry faced off against Delphi (in 1981), the Elder wand was still in the possession of its former owner, Albus Dumbledore.
For Harry to have successfully "won" it (and hence had it in his possession to face off with Delphi) would have required him to defeat the most puissant wizard in the entire world and even assuming he could have pulled off that feat (ha!), doing so would have dramatically altered the timeline, something that Harry is trying to avoid doing.
HERMIONE: Harry’s right. Time is of the essence. We need to get people into position. Now, Godric’s Hollow is not a large place but she could be coming from any direction. So we need somewhere that gives us good views of the town — that allows for multiple and clear observation points — and that will, most importantly, keep us hidden, because we cannot risk being seen.
Unknown, but it's notable that Harry wasn't at Hogwarts when he worked out what Delphi was planning. Given the anti-apparition charms that cover the estate, to recover the wand would have required him to apparate somewhere near Hogwarts, then make his way on foot to Dumbledore's grave, then make his way back the same way before finally apparating back to Godric's hollow. That would have taken a considerable amount of time that they can't spare given the urgency of the situation.
There's also the distinct possibility that if he takes the wand into the past (and is defeated) that he'd be handing Voldemort the Deathstick on a silver platter. Who knows just how crappy the world would have turned out if the Dark Lord had had the Elder Wand at the start of the Wizarding War rather than his own wand.