The Spanish league said the decision not to register Olmo, who was a youth player at Barcelona but left for Dinamo Zagreb aged 16, was initially taken by its Budget Validation Body.
It was then confirmed three times by its Financial Fair Play Committee, its Social Appeals Committee and by the Uefa Second Instance Licensing Committee of the Spanish Football Federation.
Barcelona decided on the judicial route, but now face a race against time to find an alternative plan to register Olmo or he could become a free agent, with Premier League clubs reportedly interested.
Spanish media reports say the club is planning to sell VIP boxes at its Camp Nou stadium which could bring in over 100 million euros, but it will need the approval of La Liga.
Olmo, who helped Spain win the European Championship in the summer, has scored six goals in 15 appearances for Barcelona this season.
Hansi Flick’s side are third in La Liga, three points behind leaders Atletico Madrid.
