TLDR; A Zionist organization is mad that other people don't agree with it's views, so it calls them racist.

A resolution passed by 118 votes to one on Sunday declared that the Greens "by adopting an antisemitic platform become a racist party".

Board of Deputies president Phil Rosenberg said: “By adopting an antisemitic platform, the Green Party of England and Wales has become a racist party. We will treat it accordingly.

From the Wikipedia page for the Board of Deputies:

Notably, while Lord Rothschild was President of the Board of Deputies, the Balfour Declaration was addressed to him and eventually led to the creation of a Jewish state in Palestine.

In 2018, over five hundred British Jews signed a letter from Yachad saying that the Board had "deeply misrepresented" their views after the board criticised Hamas for "repeated violent attempts at mass invasion" but did not call for Israeli restraint or acknowledge that the IDF may have acted disproportionately in massacring Palestinians at the Great March of Return. Liberal Judaism said that "the Board's credibility as the voice of British Jewry depends wholly on its willingness to listen to, hear from and reflect the values of all sections of the community".

In 2019, following Jeremy Corbyn's decision to resign as leader of the Labour Party [...] the Board asked candidates for the leadership to sign up to ten pledges in order to "end the antisemitism crisis". The pledges included a resolution of outstanding disciplinary cases, lifetime membership bans for some individuals, an independent disciplinary process, full and unconditional adoption of the International Holocaust Remembrance Alliance (IHRA) Working Definition of Antisemitism

So they're a Zionist organization. No surprise that they are against this motion. They should be ignored.