"合わないひと"
People who don't click.
People who don't match.
People who are not suitable (得に恋愛）
"相性が悪い人"
People who are not compatible / people who are incompatible.
合わない人に会った
I met a person with whom I just didn't click.
I met a person with whom I just don't match.
I met a person who wasn't really suitable for me (得に恋愛）
I met a person that I'm not really compatible with.
The person I met was not someone I could get along with.
I met a person with whom our personalities clashed.
ご質問ありがとうございます。
① "The person I met was not someone I could get along with."＝「仲良く付き合えない人と出会った。」
② "I met a person with whom our personalities clashed."＝「お互いの性格が衝突する人と出会った。」
☆ "personality clash"を名詞として使うこともできます： "We had a personality clash."
ご参考に。
モロッコ 
アメリカ合衆国 
