i am unable to login as a user using REST API, same is working fine with Administrator. Can any one please help me out in this ?

I have assigned all required permissions for user ( API Enabled, Modify all data, Connected apps and also crated connected app).

Tried with user by assigning system administrator profile, still its not working.

SF beginner here.  Ive been setup with a sandbox to work in their lightning.force.com environment.  I'm trying to obtain an access token using a windows app via http.  First thing I did was create a connected app.  I have it setup for OAuth and saved the Consumer Key and Secret codes.  When I try to use the password flow to get an access token, I get an invalid_client_identifier.  I have checked the value and it's correct.  I'm thinking it must be some sort of permission issue.  Do I have to use an endpoint in my sandbox, or do I use the documented https://login.salesforce.com/services/oauth2/token ?  Any other thoughts?