What does this ERROR message mean?

The try-catch block performs a check to see if the given index definition is literal, i.e. TEXT or ASCII.
If an exception is thrown, it defaults to 'false' - not literal type.
If the index defaults to false, this means the SASI internal data structure will treat the indexed column as numeric, i.e. int, bigint, etc..
In the worst case scenario, the user would not be able to read data via the index, or an insert statement would fail due to a mismatched data structure.

Why does this ERROR occur?

There is currently no known scenario where this error has been thrown, nor is there an expectation this would be hit by any user.

How do you fix this ERROR?

If this error is returned, attempt to rebuild the index by running "nodetool rebuild_index <keyspace> <table> <indexName>" and monitor the log for any errors.
If an error is returned during the rebuild, appropriate action should be taken based on the error.
For example, if table corruption is reported, running nodetool scrub or sstablescrub would be the resolution.