Albus believes that his father, Harry, seems to be dismayed that he ended up in Slytherin House.
Harry says to Albus quite pointedly that he would be proud of him if that did in fact happen:
“Albus Severus,” ... “you were named for two headmasters
of Hogwarts. One of them was a Slytherin and he was probably
the bravest man I ever knew.”
“But just say —”
“— then Slytherin House will have gained an excellent student,
won’t it? It doesn’t matter to us, Al.
-Harry Potter and the Deathly Hallows, Epilogue (Nineteen years later).
But in The Cursed Child a lot of Albus' problems stem from his belief that Harry isn't happy with this turn of events:
HARRY: Is that how I make you feel?
ALBUS looks at his dad, trying to figure him out.
ALBUS: I don’t think Scorpius said, but when we returned after failing to fix the first task, I was suddenly in Gryffindor House. Nothing was better between us then either — so — the fact that I’m in Slytherin — that’s not the reason for our problems. It’s not just about that.
HARRY: No. I know. It’s not just about that.
Not sure why Albus thinks this is a problem.