With a win, Barcelona would go on to have 13 points. Inter Milan could match that total with two wins, making them tied on points if Barcelona lose their last match. But Barça own the tiebreaker between them, which is a result of their direct matchup goal difference. Thus, Barcelona automatically win the group with a win.
If Barcelona draw, that would put them on 11 points. Inter could go in front with two wins, if Barcelona draw or lose their last match. However, if Inter draw or lose today, that would leave them with at most 8 points. That means Inter could only draw level on the last match day, which again, means Barcelona win the group through a tiebreaker. Thus, Barcelona win the group with a draw if Inter draw or lose their match.
Lastly, if Barcelona lose, they can still win the group. However, if they lose, Inter can draw level on points after this round of matches. Even then, Inter would have to better Barcelona’s result in the last round of fixtures to win the group. There also exists a route for Tottenham Hotspur to win the group if Barça lose today. They would need to win their last two matches and get good enough results to win tiebreakers. Improbable, but certainly possible. In summary, Barcelona cannot win the group today if they lose, but they would not be ruled out from winning it in the next round.