Do we need to put 要 after 一定 always? What does 要 do here?

My teacher in Beijing said 休息一下 doesn't always literally mean rest for "a bit" or "a little". There are a lot of expressions that native speakers add 一下 to without literally meaning "a little" - it's a way of softening a suggestion and an idiomatic way of expressing 休息 - so here can just mean "go home and rest". Anyone else been told that?

You must go home and rest is marked wrong.

You definitely need to go home and rest a bit.

You should go home and have a rest was rejected too.

You must go home and rest for a bit.

you should go home to take a break

"You have to go home and rest" was rejected

Rest " for a while" becomes wrong???