Warning! SPOILERS for The Mandalorian season 2 finale.
Now that Din Djarin has won the Darksaber from Moff Gideon, The Mandalorian cannot simply have him give the weapon to Bo-Katan Kryze. However, this insistence on winning ownership of the Darksaber may create a plot hole because this rule hasn't necessarily been strictly followed in the past. First introduced in Star Wars: The Clone Wars, the Darksaber reappears in The Mandalorian's season 1 finale in the hands of Moff Gideon. It's still unclear exactly how the former Imperial laid hands on it, but season 2 confirms its previous owner, Bo-Katan, has been hunting him down in order to get it back.
In The Mandalorian season 2 finale, "Chapter 16: The Rescue", Din has learned Gideon's location, and when he comes to Bo-Katan asking for her help in rescuing Grogu, she agrees on one condition - Gideon must surrender to her. Why she insists on this becomes clear later on as Gideon smugly explains to Din how the Darksaber can only be rightfully claimed by winning it in combat. And since it's Din and not Bo-Katan who fights Gideon, he's now its rightful owner and de facto ruler of Mandalore. However, this hasn't always been the case, and as recently as Star Wars Rebels, the rules surrounding the Darksaber appear open to interpretation.
In Star Wars Rebels season 3, Sabine Wren discovers the Darksaber among a collection of Darth Maul's possessions on Dathomir. Previously in Clone Wars, the former Sith won the blade in a duel with Pre Vizsla, the leader of Death Watch. Sabine trains with the Darksaber, hoping to use it as a symbol to rally the various Mandalorian clans against the Empire's occupation. But upon returning to Mandalore, her mother, Ursa Wren, tells Sabine she cannot truly lay claim to the Darksaber unless she wins it in combat. Later on, Sabine manages to do just that when she fights Gar Saxon, the Empire's installed ruler of Mandalore. Yet, though Sabine rightfully wins possession of the Darksaber, she chooses to instead find a leader who will unite Mandalore against the Empire, not believing herself destined for the task. Sabine goes on to present Bo-Katan with the Darksaber in Rebels season 4, and the response from the gathered Mandalorians certainly suggests she is its new master. But according to the established rules and now The Mandalorian season 2 finale, that shouldn't have been the case.
When it comes to Rebels, it's possible to interpret Sabine's winning of the Darksaber as her winning it for the cause of finding a leader to unite Mandalore. After all, why else would Ursa go along with giving the Darksaber to Bo-Katan after chastising her own daughter for not winning it originally. Sabine also isn't a figure with the same name recognition as Bo-Katan, a woman who is a renowned warrior, leader of the Nite Owls, and sister of Mandalore's last rightful leader, Duchess Satine. By most accounts, it makes sense for Bo-Katan to wield the Darksaber and use it to unite Mandalore - except by the account that may matter most, winning it.
Historically, the Darksaber is a weapon of Clan Vizsla that has been passed down across generations. It's unknown how the weapon changed hands by their traditions, meaning it's possible they didn't adhere to the combat rule. But given the warrior nature of Mandalorian culture, it isn't impossible to believe some trial by combat was required to claim it even then. And if the latter is true, then there's really little precedent for Bo-Katan being able to claim rightful ownership of the Darksaber without winning it. Meaning what happens in Rebels season 4 is either a plot hole, or Bo-Katan's claim to the Darksaber is open to dispute right from the start.
Given how Gideon mentions Bo-Katan must win the Darksaber in combat if she's ever to wield it again and her own refusal to take the blade when offered, it seems very likely the way Bo-Katan previously received the Darksaber was disputed, at least by some. Much is still unknown about the rebellion Bo-Katan leads against the Empire or the resulting destruction of Mandalore, so it's possible some Mandalorians didn't agree to follow her. Perhaps some even tried to raise Sabine up as an alternative? If because of this the Mandalorians were never truly united, then it's very believable the Empire was able to overwhelm their fractured resistance. With the Darksaber in Din's undisputed hands, however, there's another chance to unify what remains of the Mandalorians. But only if Bo-Katan is willing to accept that again the Darksaber isn't really hers, because if not and The Mandalorian finds a way to give it to her without fighting Din, it risks repeating the same mistake as Rebels.