Why am I getting the "Postal code is invalid" error?

Why am I getting the "Postal code is invalid" error?

The postal code must be in capital letters. To update the postal codes you need to follow these steps: 

a.) Go to the template section. 

b.) Open the “Shipping template” to which that product belongs. 

c.) Update the postal code then save the template. 

d.) Then go back to the “All product” section and try to publish the products on Etsy. 

For more clarity, read here.