Colin Kaepernick’s NFL-sanctioned bid to return to the league after three years in exile unraveled before it even began.

Kaepernick arrived in Atlanta on Friday to feature his abilities at a league-organized workout on Saturday that had an unlikely goal: landing him an NFL contract. Kaepernick has been a free agent since 2016, the season he led players in controversial social-justice protests during the national anthem. But suddenly, the league was offering him a showcase at the Atlanta Falcons’ practice facility, an unprecedented...