The European showpiece in Paris was twice delayed due to what UEFA initially described as "security reasons" outside the Stade de France due to crowd congestion.
Real Madrid went on to lift the trophy by defeating Liverpool 1-0 after the match started 36 minutes late at 21:36 local time, but it was events outside the ground that dominated the headlines.
Liverpool fans had complained of heavy-handed policing outside the stadium, with video footage showing tear gas being used on supporters.
UEFA blamed ticketless fans trying to force entry and fake tickets, but Liverpool demanded an investigation and European football's governing body apologised to spectators and opened an inquiry.
The UK's culture secretary Nadine Dorries suggested fans had been "treated like animals" at the match, contrary to the suggestions by French ministers that had pinned the blame on supporters.
Lallement, speaking at the French Senate on Thursday, admitted to making mistakes and acknowledged the overuse of tear gas was not necessary.
"It is obviously a failure. It was a failure because people were pushed around and attacked. It was a failure because the image of the country was undermined," he said.
French Interior Minister Gerald Darmanin also said that much of the blame lay with Liverpool fans and that 30,000 to 40,000 arrived without valid tickets.
Darmanin's claims were met with widespread backlash, with Reds' supporters group Spirit of Shankly questioning the "incompetence" of the organisation around the final.
Lallement has acknowledged that the basis for the ticket estimation was unfounded.
"The figure has no scientific virtue but it came from feedback from police and public transport officials," Lallement added. "Maybe I was wrong, but it was constructed from all the information harvested.
"Whether there are 30,000 or 40,000 people, it doesn't change anything. What matters is that there were people, in large numbers, likely to disrupt the proper organisation of the filtering.
"But that we count them precisely to within 5000, it doesn't change much."