ORA-12703: this character set conversion is not supported - oracle

Question

I am encountering an Conversion error message when running the below code:
Declare
v_t MyType;
assignment_param NVARCHAR2(10);
project_name_param NVARCHAR2(100);
document_nick_name_param NVARCHAR2(100);
doc_created_by NVARCHAR2(100);
id NVARCHAR2(10);
project_launch_date NVARCHAR2(100);
assignment_type_param NVARCHAR2(100);
temp NVARCHAR2(30000);
Begin
assignment_param := get_assignment_type(f_id);
project_name_param := get_project_name(f_id);
document_nick_name_param := get_document_nick_name(f_id);
doc_created_by := get_doc_created_by(f_id);
id := get_id(f_id);
project_launch_date := get_project_launch_date(f_id);
IF (assignment_param = 'manual') THEN
assignment_type_param := 'manually';
ELSE
assignment_type_param := 'automatically';
END IF;
v_t := MyType();
v_t.EXTEND(7);
v_t(1) := 'assignment_param='||assignment_param;
v_t(2) := 'project_name_param='||project_name_param;
v_t(3) := 'document_nick_name_param='||document_nick_name_param;
v_t(4) := 'doc_created_by='||doc_created_by;
v_t(5) := 'id='||id;
v_t(6) := 'project_launch_date='||project_launch_date;
v_t(7) := 'assignment_type_param='||assignment_type_param;
temp := Replace_Email_Body_Contents(12,v_t);
End;
Also my other function has code:
CREATE OR REPLACE FUNCTION Replace_Email_Body_Contents
(
id_inparam IN NUMBER,
replace_var_loop IN MyType
)
RETURN NVARCHAR2
AS
db_string NVARCHAR2(10000);
str_1 NVARCHAR2(100);
str_2 NVARCHAR2(100);
BEGIN
--loading the email template from html string column of form_templates_data table;
SELECT HTML_STRING INTO db_string FROM TABLE WHERE ID = id_inparam;
FOR i IN 1..replace_var_loop.COUNT LOOP
SELECT SUBSTR(replace_var_loop(i), 0, INSTR(replace_var_loop(i), '=')-1) INTO str_1 FROM DUAL;
SELECT SUBSTR(replace_var_loop(i), INSTR(replace_var_loop(i), '=')+1) INTO str_2 FROM DUAL;
db_string := REPLACE(db_string , str_1 , str_2 );
END LOOP;
dbms_output.put_line(db_string);
RETURN db_string;
END;
/
Error:
ORA-12703: this character set conversion is not supported
ORA-06512: at "REPLACE_EMAIL_BODY_CONTENTS", line 13
ORA-06512: at line 32
HTML_STRING is a column in the table referred in above code and the values corresponding to f_id looks like:
Notification
Hello,
You have been assignment_param-assigned a document.
Project Name: project_name_param
Document Nickname: document_nick_name_param
Doc Created By: doc_created_by
LCA ID: lca_id
Project Launch Date: project_launch_date
Based on the evaluation of the checklist, an Engineer has been assignment_type_param assigned to work with the project team.