If this means 6 La Liga teams qualify, then the 4th place team in La Liga instead plays in the UEFA Europa League, as any single nation is limited to a maximum of 5 teams. The 5th place team in La Liga and the winner of the Copa del Rey also qualifies for the subsequent season's UEFA Europa League group stage.
Barcelona are competing in the Europa League this season instead of the Champions League because they failed to qualify for the knockout round, finishing third in Group E.