WooCommerce License Keys allows for errors output format configuration. This setting is located at “Dashboard” -> “WooCommerce” -> “Settings”, “Advanced Tab” -> “License Keys API sub tab”.

Grouped by property name
When “Grouped by property name” is selected, the errors returned in the response are grouped by a property name. (i.e. ‘store_code’, ‘license_key’ or ‘sku’).
Response example:
{ "error": true, "status": 500, "errors": { "license_key": [ "Invalid license key." ] } }
Associated by an error code
When “Associated by an error code” is selected, the errors returned in the response are associated by an error code.
Response example:
{ "error": true, "status": 500, "errors": { "3": [ "Invalid license key." ] } }
List of error codes
This is the list of error codes returned by the regular and paid plugins (title format [error code]:[error message]):
1: Invalid code.
Returned when the store code provided do not match the one set for the API.
2: Invalid license key.
Returned when the license key provided do not match the license key string format established by the API. (regex validation)
3: Invalid license key.
Returned when the SKU provided do not match the product SKU associated with the license key.
4: Invalid license key.
Returned when the license key provided do not match the license key string format established by the API. (missing dash validation)
5: Invalid license key.
Returned when the license key provided was not found in the database.
100: Required.
Returned when no SKU was provided in the request.
101: Required.
Returned when no license key code was provided in the request.
102: Required.
Returned when no store code was provided in the request.
103: Required.
Returned when no activation ID was provided in the request.
104: Required.
Returned when no domain was provided in the request.
200: License key has expired.
Returned when the license key provided has expired.
201: License key activation limit reached. Deactivate one of the registered activations to proceed.
Returned when the license key provided has reached the limit of activations allowed.
202: License key domain activation limit reached. Deactivate one or more of the registered activations to proceed.
Returned when the license key provided has reached the domain limit allowed.
203: Invalid activation.
Returned when the Activation ID provided was not found.