The ruling body said the only way for the Argentine forward to be released from the contract was to trigger a 700-million euros (NZ$1.2 billion) release clause.
The 33-year-old six-times world player of the year informed his lifelong club last week he wished to leave immediately, plunging Barca into new turmoil less than two weeks after their humiliating 8-2 Champions League quarter-final defeat by Bayern Munich.
Messi's lawyers plan to invoke a clause in his four-year contract, signed in 2017, which would have allowed the forward to leave the club for free if he had requested it by June 10.
They will argue that that date - nominally the end of the season - is now irrelevant after the coronavirus delays that led to the season's extension and the team playing deep into August.