If you are running Magento version 2.4.4 or above, ensure the following option is set to Yes:
Stores > Configuration > Services > OAuth > Consumer Settings >
Allow OAuth Access Tokens to be used as standalone Bearer tokens
If this option is set to No, API requests to Magento may fail with errors such as:
The consumer isn't authorized to access %resources.
Note: This option defaults to No, and may be introduced after upgrading Magento from a previous version. So if you've recently upgraded Magento, please ensure this option is set to Yes.
Next: Gather API Credentials