SQL & PL/SQL :: Writing A Code For When Condition

I have a problem in running a sql query.I have a dataset with the following details. Product name,product status,approval date in table product_details. I have a code as follows

select Product_name,
Product_status,
approval_date,
case
when product_status ='Cancelled' or product_status ='Stopped' then approval_date='N/A'
when product_status='Active' then NVL2(approval_date,cast(appoval_date as nvarchar2(30)),'Null')
when product_status='Completed' then NVL2(approval_date,cast(appoval_date as nvarchar2(30)),'Null1')
when product_status='Planned' then NVL2(approval_date,cast(appoval_date as nvarchar2(30)),'Null2')
end as DER_approval_date

from product_details

but i have a error in running this code saying character mismatch.