You must set up the OAuth and OpenID flows for your connected app. 

If you turn on this setting, then the whole org is affected.

To enable OAuth settingsLink to To enable OAuth settings

  1. Navigate to Setup > Settings > Identity > Oauth and OpenID Connect Settings. A list of flows is displayed. 
  2. Turn on the Allow OAuth Username-Password Flows setting, if it is not enabled.

Next Step: Creating a Connected App