John Gill Commentary Numbers 19:20

"But the man that shall be unclean, and shall not purify himself, that soul shall be cut off from the midst of the assembly, because he hath defiled the sanctuary of Jehovah: the water for impurity hath not been sprinkled upon him; he is unclean." — Numbers 19:20 (ASV)

But the man that shall be unclean
By touching any dead body, bone, or grave:

and shall not purify himself ;
with the water of purification:

that soul shall be cut off from among the congregation :
(See Gill on Numbers 19:13).

because he has defiled the sanctuary of the Lord :
by going into it in his uncleanness:

the water of separation has not been sprinkled upon him, he is
unclean ;
and will remain so, for nothing else could purify him, see (Numbers 19:13) .