No, Jack Smith could not release his final report on his own. As a special counsel appointed by the Attorney General (AG), he was required by regulation to submit the report confidentially to the Attorney General, who then decided whether and how much of it to make public.
Attorney General Merrick Garland followed this procedure, but the public release of the report was complicated by ongoing legal battles and court orders:
Submission Process: Special counsel regulations dictate that at the conclusion of the investigation, the special counsel provides the AG with a confidential report explaining the prosecution or declination decisions.
AG's Discretion: Garland made public the reports from other special counsels (Robert Hur and John Durham) in their entirety and intended to do the same with Smith's report to inform Congress and the public.
Legal Challenges: Lawyers for Donald Trump and his co-defendants sought to block the report's release, arguing it contained prejudicial information and that Smith's appointment was unconstitutional.
Court Rulings: A federal judge, Aileen Cannon, temporarily blocked the release of the entire report while an appeal regarding the co-defendants was pending. An appeals court later cleared the way for the release of the volume concerning the election interference case, but the volume on the classified documents case remained under a court-ordered prohibition from public release while charges were still technically pending against co-defendants.
Public Release: The Justice Department ultimately released the volume of the report on election interference, while agreeing not to publicly release the documents case volume out of fairness to the co-defendants.
https://tinyurl.com/4d6rht4p
For the record, Robert Mueller couldn't release his Russian collusion report either without the AG's approval. James Comey also had to get approval to release his report on Hillary's e-mails.