I got the unauthorized_client error in the process of getting oAuth

I use the Less Secure method to get the oAuth ,
but I can’t get the oAuth.

I use postman to send post request as follows.

And the client_id ,client_secret is my partner’s, the email_edress and password are mine. I dont’t know whether it matters.

it looks like your authorization is not correct. it should not be the client ID and client secret as username and password as authorization, it needs to be an Authorization header as described here:

OAuth Login - Developers - Sketchfab

Authorization HTTP header

Authorization: Basic CREDENTIALS

CREDENTIALS is the base64 encoded string of your app credentials, separated by a colon, i.e. base64(client_id:client_secret).