PSG, who were without the league's top scorer Zlatan Ibrahimovic through suspension, suffered their second straight home loss despite the visitors being reduced to nine men early in the second half.
The mega-rich capital club, who have not won in their last three league games, had numerous chances to score but they wasted good positions by preferring to shoot rather than pass to team-mates.
"I am here to build a team, which we are not yet," the Italian coach told a news conference.
"It is not acceptable to play like we did tonight. It is not acceptable to lose two home games in a row. Some things will change for the next games," he added.
"It is a huge disappointment. We are not a solid team yet, we do not have a good attitude. Our problem was psychological. Some things will change."
PSG, who were top on 30 points from 13 games last season before Ancelotti was brought in and eventully lost the title to unheralded Montpellier, are only on 23 at the same stage this term.
This comes despite having spent millions to sign players such former AC Milan front-man Ibrahimovic and centre-back Thiago Silva during the off-season.
"Ibra", who is top scorer with 10 goals from 10 league matches played and wowed the world with his overhead-kick for Sweden in midweek, also had to turn himself into a playmaker in PSG's last home Champions League game against Dinamo Zagreb when he set up all four goals (4-0).
‘He instantly popped into our ratings as one of the best U21 midfielders in Europe. The impressive thing is Slot has made him into more of a no.6' Inside Ryan Gravenberch's transformation at Liverpool
‘I don’t think Liverpool would look at Ollie Watkins, a striker isn’t a pressing issue for them – it’s Arsenal who need one’ Former Reds star explains why his old club don’t need an out-and-out forward this summer
