The Argentine superstar announced on Friday he had been forced to remain at Barcelona, claiming after club president Josep Maria Bartomeu broke his word to let him leave.
It came after 10 days of tension since his bombshell fax telling Barcelona he wanted out.
But while other players including Jordi Alba and Philippe Coutinho arrived for the session, Messi failed to appear at the Joan Gamper training complex for the session which began at 0730 GMT. Last Sunday Messi deliberately skipped the mandatory coronavirus test for Barcelona players.
According to local media, Messi must first pass the Covid-19 test with a return to the squad possibly on Monday.
He could then figure in new coach Ronald Koeman's first fixture, a friendly against third division Nastic on September 12.
