I am trying to set up a custom field that is a checkbox which is going to be true if all the checkboxes are true at line level below or it wont get checked in the checkbox if even one of the values at line level has the checkbox unticked(due to line level logic)

I tried to set that field as a read only field and wrote below script ( I am using using select statement as Implementation type) -

select CASE 
WHEN((select count(CF$_TEST) from PART_AVAIL_STRUC1931493261_CFV J WHERE CF$_TEST = 'False' AND J.TOP_CONTRACT = :TOP_CONTRACT AND J.TOP_PART_NO = :TOP_PART_NO) > 0) THEN 'FALSE' ELSE 'TRUE' END FROM DUAL

Now when I finish the process by selecting field type as enumeration and boolean where enumeration is FndBoolean I get the attached error Can somebody help me understand, where is the issue what I might be doing wrong ?