Grants

A grant is a class implementing the grant interface, tagged in the wiring; the token endpoint holds no match on a grant type, and the discovery document announces exactly the grants that are tagged. What a grant can do is declared by the interfaces it implements: whether it sends a person to the client, whether it issues refresh tokens, whether it acts for the client itself, whether it exchanges a token.

Grant type	Page
authorization_code	Authorization code
refresh_token	Refresh token
client_credentials	Client credentials
urn:ietf:params:oauth:grant-type:device_code	Device code
urn:openid:params:grant-type:ciba	Backchannel authentication
urn:ietf:params:oauth:grant-type:token-exchange	Token exchange
urn:ietf:params:oauth:grant-type:jwt-bearer	JWT bearer

The resource owner password credentials grant is not in that list and never will be.