Amazon athena, presto sql :INVALID_FUNCTION_ARGUMENT: Cannot unnest type: varchar

Have some transaction data and the data looks like:
record_id labels
001 ['first_record','transfer']
002 ['withdraw', 'holiday']
003 ['direct','change_password','transfer']
004 ['change_password']
......



Labels column is of string type.


Tried to unnest labels column
select record_id, new_labels from transactions, UNNEST(labels ) as t(new_labels ) order by record_id, new_labels



but got following error



INVALID_FUNCTION_ARGUMENT: Cannot unnest type: varchar



Then tried to cast string to array then to the unnest
select record_id, cast(json_parse(labels) as array(varchar)) as labels from transactions



but got following error:



INVALID_CAST_ARGUMENT: Cannot cast to array(varchar). ['withdraw', 'holiday']



Then simply tried
select cast(['first_record','transfer']) as array(varchar)



it returned



mismatched input '['. Expecting: