It might be the case of not proper configuration of your application server/database/other middleware, for example if the number of concurrent users is higher than the maximum number of connections in the pool - the remaining ones will "wait" until the next connection becomes available for serving them
So double check your application configuration ...
it is not a JMeter error. the user you are using to perform action doesn't have access. I say it is application things rather than JMeter.
Moreover, you can compare the request headers with another one and find the difference.
With JMeter you can organize your test plan as follows:
HTTP Request to send an API call
While Controller to wait until the data is available in the database
JDBC Request sampler to periodically check the database for updates
Optionally a Transaction Controller to measure the whole workflow duration
Example test plan outline: