。
You must not be mean to your friends.
Literal
Friend [target-に] meanness [object-を] do [must not-てはいけません].
意地悪をする ('to do meanness') is a noun+する construction where 意地悪 names the behavior rather than describing it adjectivally. ~てはいけません is the polite prohibition form, often heard in classroom or parenting contexts.