Description
The error will display when the NetSuite concurrency limit for your account is exceeded.
Cause
The default concurrency limit (or maximum number of integration terminals) for a NetSuite account is 5. This means that at maximum of 5 requests by any integration can occur at the same time. This does not mean there can only be a maximum of 5 integrations. Think of it like a road with 5 lanes and only 1 car can be in a lane at a time. If all lanes have cars and a new car tries to enter the road, then the new car is rejected and can try again once a lane becomes available.
Solutions
Try again.
If this happens frequently, you can contact NetSuite Support to see if your concurrency limit can be increased.