select collect(column_name) from user_tab_columns
The problem the tool is having is that it is getting back a data type which it doesn't recognize.
If you don't already have a defined collection type, then go create one as they are really useful. This is mine
CREATE OR REPLACE TYPE TAB_CHAR IS table of VARCHAR2(4000);
Then you can do an explicit CAST in the select statement and the error will go away.
select cast(collect(column_name) as tab_char) from user_tab_columns