Use of null in case expression
You need to remove the comma from this part of your code:

CASE 
  WHEN state IS NULL, THEN 'N/A'
  ELSE state
END


The correct version would be:

CASE 
  WHEN state IS NULL THEN 'N/A'
  ELSE state
END


You can also use the coalesce function:

COALESCE(&lt;VALUE&gt;,&lt;OUTPUT IF NULL&gt;)   


The COALESCE function translates well this code:

CASE WHEN &lt;value&gt; IS NOT NULL
     THEN &lt;value&gt;
     ELSE &lt;value_2&gt;
     END


Coalesce returns the first non null value in the list.

Using COALESCE, your query will appear like this:

SELECT 
    customerNumber,
    customerName,
    COALESCE( state, 'N/A') as state,
    country
FROM
    customers;