What does this ERROR message mean?

This error means a node was able to successfully start up and move to the normal stats (UN) but has no tokens assigned and therefore will not get any data.

Why does this ERROR occur?

The safeguards and automation built into DSE/Cassandra should effectively make reaching this error impossible. However, for extreme legacy version of Cassandra it may be possible to misconfigure the node to not own any tokens.

How do you fix this ERROR?

The safest path to fixing this error is to decommission the node (to inform the remaining nodes that it is no longer a part of the cluster), wipe all the data files, commit log, etc. and shut it down. You can then start the node up again to attempt joining the cluster again. Since this should never happen in theory, you should never encounter this issue. If you do, please notify DataStax Support so we can file a JIRA for the bug.