Most probably this error happened because you are using a Mobile authentication key within your Web integration.
In this article you will be going to know about:
How to investigate the reason that caused this error?
You can investigate the reason for such an error from the "API debug log" by navigating to the Developer’s menu > Testing > API Debug Logs (within your desired profile).
Or you may get the error directly from the response payload sent according to your sent request
Reason #1: Using a Mobile authentication key within Web integration.
This means that the server and the client key you are using in your integration are not for Web usage, and you are using Mobile authentication keys instead as shown in the picture:
In this case, you have to switch the key used to the correct one. If you don't have Standard Web authentication keys, you can always create a new one from the (+) icon. To know more about this please check our How to get my Authentication/Integration/API Keys? solution article.