"당신은 예뻐요" is correct。"예쁘다"is an adjective "ㄹ/을 " + Verb

당신은 예뻐요. = You are pretty. (Correct) 당신을 예뻐요. (Wrong) because 당신은 -> subject 당신을 -> object so '당신을 예뻐요' needs the subject we can say 나는 당신을 사랑합니다. = I love you