X: It's a pity that you were absent from the training session.
O: It's a pity that you missed the training session.
O: It's a pity that you weren't at the training session.
Be absent = Not be present at something that you were officially supposed to attend. "Her teacher wanted to know why she had been absent from class."
Miss: Not be present at something. "I wouldn't miss her party for the world!"
Be at = To be present at something. "John won't be at the meeting."
