I have the same Error message. Inventory is enabled, and Authorization header is set to 'OAuth'.

using the swagger in https://glpi.myserver.XX/api.php/doc works

Error:
"title":"You are not authenticated","detail":"The Authorization header is missing or invalid","status":"ERROR_UNAUTHENTICATED"

the OAuth client has every Grants and every Scopes configured.

We are on version 11.0.1