This was debunked back when it came up in 2017 (?) There’s a good article about it here. The summary is that Stein was there in a diplomatic role. You know, as presidential candidates do. The Senate Intelligence Committee investigated her and found no wrongdoing.

A campaign spokesperson told Newsweek that Stein “attended at her own expense to spread a message of peace and diplomacy” and gave a speech in Moscow “in which she criticized the excessive militarism of both Vladimir Putin and U.S. leaders.”

Speaking to The Intercept in 2017, she said the notion that it was an “intimate roundtable” was “mythology,” and that Putin and his associates “weren’t at the table for very long.” Stein said that “nobody introduced anybody to anybody” and that she “didn’t hear any words exchanged between English speakers and Russians” due to the lack of a translator.

Stein said that Putin had appeared to make a speech and left immediately after. “Nobody cared to make introductions. This wasn’t intended to be a discussion of any sort,” she told the outlet.