Could help to understand the issue? I am using this endpoint to get an access token https://www.wikidata.org/w/rest.php/oauth2/access_token?client_credentials=client_credentials and providing client id and client secret
{
"error": "access_denied",
"error_description": "The resource owner or authorization server denied the request.",
"hint": "Client 4ab9e80d07a34633cdeab291fd8ead6a is not usable by user with ID 0",
"message": "The resource owner or authorization server denied the request."
}