What does this error mean?

This error indicates that a partition key length is greater than the CQL limit of 65535 and is rejected when trying to append the key to the row index.

Why does this error occur?

This error occurs when an application attempts to write a record where the partition key length is greater than 65535.

How do you fix this error?

You will need to adjust the partition key in order to make it fall within the CQL limits. See for reference.