521 ERROR Destination postal code missing or invalid [Solution]

When processing orders you might come across the "521 ERROR Destination postal code missing or invalid" error.

This error is shown when the Postal Code used in an order does not have a correct format or is missing.

Feel free to follow the step-by-step guide written below or the video guide, as you prefer:
 

NOTE: To verify if a Postal Code is valid, do not rely on user-fed/collaborative services like Google/Bing Maps, since they are prone to errors or mismatches.
For official confirmation use government-owned sources (Canada Post for Canada, USPS for USA, Royal Mail for Great Britain, etc).

To do so:

1. Find the order through any of the possible ways (after Import Orders, Batches, Order History, etc) as it applies to the case
 

2. Click the edit button of the order you wish to change

3. Change or Add the Postal Code depending on the case

4. Press Save to apply the changes

If the rest of the information in the order is correct, you will be able to process it.