As Muns says it is difficult to find an equivalent expression to 'sucks' in Arabic. Bear in mind that 'it sucks' is even slang not formal English. So it could be expressed in various ways (an expression of disdain and it wont be a literal translation in all cases.) Since it is a play, mentioning a whole paragraph will help to choose the most appropriate expression to use in that context. Some possible expressions: أنت لا تطاق أنت مقرف أنت شخص غير محتمل إلخ

actually, i cannot find a phrase like this in arabic, but i think this gives the same meaning "أنت مقزز".

The teacher's answer above is the best, can't think of any better!

in your 'acting' class ?

Did you ask how to 'say' it ?