That's the claim of a letter sent to the Biden campaign by 50 or so liberal organizations, who allege he will not get that enthusiastic support unless he embraces a lengthy series of demands included in their letter. These demands include reparations, defunding the police and withdrawing support for investments in community policing.
There are two things wrong with the letter's argument. First, Biden already has sufficient black support to win the election, even though he is currently running a little behind Clinton's 2016 pace. Moreover, recent data suggest that support is firming up (see below). The fact of the matter is that Biden is solidly ahead at the moment because he has reasonable nonwhite support and is running way ahead of Clinton among both white college and noncollege voters. He would benefit from increased black support relative to the current baseline--the more votes and the bigger margin he has, the better--but he does not "need" that to win the election.
Perhaps what the letter authors mean is that Biden cannot afford a catastrophic decline in black support and/or turnout relative to 2016. But that is not what the authors said and there is no evidence at the current time that he is risking that.
And we certainly have no evidence that embracing the demands of the letter would forestall such a decline or even increase black support much beyond where it already is. Much of what they advocate is not only not popular overall but is also not popular among black voters specifically. In contrast, the reforms Biden has already embraced are all wildly popular among black voters
Hopefully, Biden will ignore this letter and concentrate on the broad coalition he has assembled which does not fixing in the ways advocated by the letter writers.