Pivot query result in MySQL

Question

With the following query I have created a very simple result with two rows and two columns:

SELECT
a.type,
AVG(DATEDIFF(a.completed, b.created)) AS avg_time_delta
FROM 
a
INNER JOIN a ON b.record_id = content.id
INNER JOIN b ON c.client_id = clients.id
WHERE
record_type = "content"
AND
c.id = "999"
AND a.completed IS NOT NULL
AND b.type = "article"
AND b.spiked = 0
AND (a.type = "TCE sign-off" OR a.type = "Final sign-off") 
GROUP BY a.type

|         type        |  avg_time_delta  |
|---------------------|------------------|
|    Final sign-off   |         20       |
|     TCE sign-off    |         10       |

I am trying to transpose this result so that the columns are ["Final sign-off", "TCE sign-off"] and there is a single row of [20, 10]. I am really struggling to do this in MySql without errors. Here is my attempt, any pointers would be appreciated.

SELECT * 
FROM (
    SELECT
    a.type,
    AVG(DATEDIFF(a.completed, b.created)) AS avg_time_delta
    FROM 
    a
    INNER JOIN a ON b.record_id = content.id
    INNER JOIN b ON c.client_id = clients.id
    WHERE
    record_type = "content"
    AND
    c.id = "999"
    AND a.completed IS NOT NULL
    AND b.type = "article"
    AND b.spiked = 0
    AND (a.type = "TCE sign-off" OR a.type = "Final sign-off") 
    GROUP BY a.type
) AS source_table
PIVOT (
    avg_time_delta
    FOR type in ("Final sign-off", "TCE sign-off")
) AS pivot_table