To Complain
する verb
You know how 文句 is a complaint, right? Well, add する to make it a verb and you have to complain. This is a really casual way to complain though, usually used in manga or on social media. Keep your eye out for it!
もんくする
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
意見を言っただけなのに、上司に「文句するな」と怒鳴られました。
I was just saying my opinion, but my boss yelled, "don’t complain," at me.