New King James Version
So He said to them, “How is it you do not understand?”

King James Bible
And he said unto them, How is it that ye do not understand?

American Standard Version
And he said unto them, Do ye not yet understand?

Berean Study Bible
Then He asked them, “Do you still not understand?”

Douay-Rheims Bible
And he said to them: How do you not yet understand?

English Revised Version
And he said unto them, Do ye not yet understand?

World English Bible
He asked them, "Don't you understand, yet?"

Young's Literal Translation
And he said to them, 'How do ye not understand?'

Mark 8:21 Additional Translations ...