The most senior police officer on duty before the Manchester Arena attack had taken an “unacceptable” two-hour break before the bombing, the inquiry heard.
PC Jessica Bullough admitted she then missed bomber Salman Abedi walking from the train station into the arena.
The British Transport Police (BTP) officer had been qualified for only eight months, and was still in her probationary period.
The suicide bombing killed 22 people and injured many more on 22 May 2017.
The public inquiry into the attack heard there were no police officers on patrol when 22-year-old Abedi made his journey from Victoria Station to the arena foyer.
The hearing was told PC Bullough took a break of two hours and nine minutes, during which time Abedi walked from the tram stop into the City Room.
PC Bullough admitted her break should have been between 50 minutes and one hour.
The inquiry heard if she had come back 10 minutes earlier she would have seen Abedi carrying a large rucksack that contained explosives.
She said looking back, it was “unacceptable” to have taken a break of that length, and said she probably would not have done that had a supervisor been present.
PC Bullough was the first on the scene in the foyer after the suicide attack at the end of the Ariana Grande concert.
She said: “I think the training I had wasn’t sufficient to deal with what I was witnessing.”
Guy Gozem QC, representing some of the bereaved families, said: “Effectively did you feel left in the lurch?”
“Yes,” she replied.
BTP PCSO Lewis Brown said he and a colleague took a break before other officers had returned from theirs, meaning there were no one on patrol between just before 21:00 and 21:35 BST, when Abedi made his trip from the station into the foyer.
Meanwhile a father picking up his children on the night of the Manchester Arena attack told the inquiry he thought “straight away” that Abedi was a suicide bomber.
Neal Hatfield said when he saw Abedi in the foyer of the arena his rucksack did not look normal as it did not flex under his weight.
“It was rock solid, that’s what alarmed me straight away,” Mr Hatfield said.
Mr Hatfield was about to go up the stairs to the mezzanine area of the arena’s City Room when he saw Abedi with his back to him “in the process of lying down, he had a backpack on the floor next to him”.
“I thought suicide bomber straight away, very little doubt in my mind. Honestly, my heart was racing,” he said.
“The way he was dressed, the way he was acting, the body language was that he was trying to protect the bag. He was pretending to be casual, but I could see what he was doing.”
Mr Hatfield said he made eye contact with Abedi, who looked “emotionally distressed”.
“He seemed frightened, his eyes were glazed over and he seemed nervous, agitated, he didn’t seem right,” he added.
Mr Hatfield told the inquiry he saw two members of the security team nearby and believed they were having a conversation about Abedi, and gesturing towards him.
The inquiry continues.