It's very helpful to see the actual error message.

You may also consider selecting

JSON_VALUE(ecommerce, '$.transaction_id')

in bigquery directly to make sure you see what it is returning.

if the transaction_id field is really

NULL

then you may instead want to use:

SELECT
  user_id,
  event_timestamp as timestamp
FROM
  `growthbook-ga4-396012`.`analytics_308238632`.`events_*`
WHERE
   event_name = 'purchase'  
  AND JSON_VALUE(ecommerce, '$.transaction_id') IS NOT NULL

because you may have gotten the following error:

Operands of != cannot be literal NULL

So you probably just need to use

ecommerce.transaction_id

instead of any of the JSON_VALUE extraction logic

Try:

SELECT
  user_id,
  event_timestamp as timestamp
FROM
  `growthbook-ga4-396012`.`analytics_308238632`.`events_*`
WHERE
   event_name = 'purchase'  
  AND ecommerce.transaction_id IS NOT NULL