I took the time to copy and paste directly from the article so you won't have to work to educate yourself. The NBPA is the players union which speaks on behalf of the players.
Directly from the aforementioned article:
The NBA and NBPA continue to negotiate aspects of COVID-related protocols and procedures for the upcoming 2021-22 campaign, but the NBPA has refused to budge on its demand that players not be required to take the vaccine, sources say, and any proposal that mandates vaccination remains a "non-starter."