You should relax more!　 你要放松一点！／你应该放松一点！( “要”can be used to give someone advice.) You must relax more!!!!!!　 你一定要放松一点!!!!!! (“必須”is used more often in written form.)

another case of using English grammar in Chinese language "You should relax more!" "You must relax more!!!!!!" What makes you think Chinese people use "should" in such sentences? There is no "should" and "must" in Chinese culture unless it's a job is relaxing a job? unlikely therefore 1 多多休息 2 注意休息 and try to imagine how weird "pay attention to relaxing" looks in English this is how "should" and "must" look in Chinese

"You should relax more!" yao "You must relax more!!!!!!" bi xu

you should is '应该‘ you must is “必须”