Without Pay, Gratuitous, Free Of Charge
noun
You already learned that 報酬 means "compensation," so adding not to it means the person working isn't getting paid, aka working without pay.
むほうしゅう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
無報酬でもかまいませんよ。
There’s no pay, but I don’t mind.