I read this in a company’s employee-education course about a week ago.

Problem:
The word “great” is not an adverb.

Explanation:
The word “great” in “You have done great.” acts as an adverb because it modifies the verb “done”.

Writing “You have done great.” is just as wrong as writing “You have done good.” because neither the adjective “good” nor the adjective “great” should be used to modify a verb.

The solution comes from replacing the adjective “great” with an adverb that carries a similar sentiment.

Solution:
“You have done well.”