Shall not purify himself, i.e, shall contemptuously refuse to submit to this way of purification.
Related Commentaries of Numbers 19:20
Numbers 19:20
20 But the man that shall be unclean, and shall not purify himself, that soul shall be cut off from among the congregation, because he hath defiled the sanctuary of the LORD: the water of separation hath not been sprinkled upon him; he is unclean.