Full ERROR Message Example:

ERROR 2020-10-06 10:50:57,640  PerSSTableIndexWriter.java:321 - Failed to flush index {cql_test_keyspace.table_45.table_45_v1_idx} /var/lib/cassandra/data/cql_test_keyspace/table_45-69e31260d03311eabb304535be539fe2/ba-2-bti...

What does this ERROR message mean?

It means there is some implementation error in SASI or the indexed value violates the SPARSE definition. See:
https://docs.datastax.com/en/dse/5.1/cql/cql/cql_using/useSASIIndex.html#SPARSE%20index

Use SPARSE indexing for data that is sparse (every term/column value has less than 5 matching keys)

Or any other generic unhandled error.

Why does this ERROR occur?

If the index is created but doesn't adhere to the SPARSE definition, this error will occur.

How do you fix this ERROR?

An index rebuild is needed if the error is recoverable. However, all related index files get deleted, and the base table is not affected. Index reads will not return all results, though.