ORA-02250 / Missing Or Invalid Constraint Name

I wrote a function:

create or replace function get_search_condition(p_cons_name in varchar2 ) return varchar2 authid current_user is
l_search_condition user_constraints.search_condition%type;
begin
select search_condition into l_search_condition
from user_constraints
where constraint_name = p_cons_name;
return l_search_condition;
end;

and ran it...then when I write the below command:

alter table employee drop constraint
select constraint_name from user_constraints where table_name = 'EMPLOYEE' and
get_search_condition(constraint_name) like '%(( ROLL_NUMBER > 0 ))%'
;

The system throws:

Error starting at line 1 in command:
alter table FL_ITEMSITE drop constraint
select constraint_name from user_constraints where table_name = 'FL_ITEMSITE' and
get_search_condition(constraint_name) like '%(( SS_QTY > 0 ))%'
Error report:
SQL Error: ORA-02250: missing or invalid constraint name
02250. 00000 - "missing or invalid constraint name"
*Cause: The constraint name is missing or invalid.
*Action: Specify a valid identifier name for the constraint name.

i want to drop constraint without specifying its name. i want the system to fetch it dynamically.