Nottingham forest were set to qualify for Champions league but they were not able to perform well towards the end of 2024/25 EPL season. Nottingham forest finished 7th last season and they must be happy that they atleast qualify for UEFA conference league. That's a big achievement for a team that finished just one spot over the relegation zone in 2023/24 season.
United finished 15th last season and as I said if Nottingham forest can make a comeback in next season then any team can do that provided they identify their weaknesses and fix them.
Nottingham Forest performed well last season, dropping from 3rd to seventh place. I was surprised when they dropped back from third place, but not everyone on the team has the strength to maintain their winning throughout the season. You are right, if Nottingham Forest, who finished at bottom of the league last two seasons, could finish seventh last season, then Manchester United can achieve the same thing if they fix their problems.
However, Nottingham Forest will not compete in the Conference League next season, but in the Europa League. Crystal Palace, the FA Cup winners last season, are going to be demoted to the Conference League since winning the FA Cup usually qualifies you for the Europa League, but FIFA has decided to demote Crystal Palace, while Nottingham Forest will compete in the Europa League.