What is Beta Record? I tried access inventoryAdjustment and customerPayment and get the error notification from Postman : "The record is only available as a beta record. Enable the REST Record Service (Beta) feature in Setup > Company > Enable Features to work with this record"

That means that the REST API implementation for that record is still in Beta, not Generally Available or production ready. Turning on the feature requires agreeing to a beta feature agreement, and allows you to use the Beta parts at your own risk. You may just want to leave the feature turned off and allow the system to continue stopping you from using the parts of the REST API that aren't finished yet.

Thank you for the explanation