Pompey missed out on a play-off spot after ending the League One season with a home defeat to Accrington.
The 1-0 reverse ensured that they were leapfrogged by Oxford and Charlton, finishing the 2020/21 campaign in eighth position.
Adam Phillips netted the only goal of the contest midway through the first half and although Danny Cowley's men fought back, they could not turn the contest around.