Brock Lesnar isn't currently part of WWE's plans for SummerSlam 2021.
Per Dave Meltzer on the Sunday Night's Main Event podcast, WWE had not called on the former WWE and Universal Champion as of last week, with the decision ultimately coming down to Vince McMahon.
Some people within WWE don't want Lesnar to return until Universal Champion Roman Reigns is deemed ready for a feud with him. This would be a SummerSlam or WrestleMania-calibre match, particularly with how well-received Roman's current run has been, but Reigns has been linked to a bout with John Cena at this year's Biggest Party of the Summer.
Lesnar vs. Bobby Lashley had been speculated for SummerSlam. The current WWE Champion has never been shy of expressing his desire to face Brock, though this looks like a non-starter at the moment. Plans change, though.
Lesnar hasn't been on WWE television since dropping the WWE Championship to Drew McIntyre on WrestleMania 36's second night in April 2020. He became a free agent that August, as his previous contract expired, though Paul Heyman recently hinted that his (former?) client could return the next time WWE hits Houston - 16 July.