Korean is unusual in that its "adjectives" (or "stative verbs") behave like full-fledged verbs. (Imagine, instead of "He is happy" or "a happy person", you say "He happies" and "a happying person".)
Standard Korean used to disallow imperatives for adjectives, so, for example, instead of saying "행복해라 Be happy", you would use "행복해져라 Become happy" or other similar construction. However, these days, many adjectives are used with imperatives freely, though I think it's still frowned upon by prescriptivists.
One place imperatives are still not allowed is actual copula "-이다 ida", which attaches to a noun. E.g.,
그는 행복한 사람이다. He is a happy person.
geu-neun haengbok-ha-n saram-i-da
He-topic [happiness-(adj making suffix)]-(modify following noun) person-be-(sentence ending)
You cannot change it to "Be a happy person", so instead you will need a different verb, such as "Become a happy person", or maybe "Live happily", or something else.