No Interest, Not Interested
expression
興味 is interest in something. If you do not have (がない) that interest, then you have no interest or you're not interested.
きょうみがない
You learned that the word 興味 uses the on'yomi readings for both kanji, so all you have to do is add が + ない and you've got a new expression!
俺は顧客と個人的に仲良くなることには興味がないんだ。
I’m not interested in building personal relationships with customers.