Why can I not use the RCS chats?

When you use the RCS chats, your messages are sent with the Rich Communication Services(RCS) protocol over Wi-Fi and mobile data. RCS chats will be turned on only when all the participants in a conversation have RCS. RCS chats are provided by Google or your mobile carrier.
To check your RCS availability and status:
1. Go to the [Messages] APP on your device;
2. Tap [Profile] in the upper right corner and go to [Messages settings];
3. Go to [RCS chats].
To troubleshoot your RCS chats connection:
1. Update Google Messages and your Carrier Services from Play Store;
2. Make sure Google Messages is set as your default SMS APP;
3. If your device has dual SIM slots, and both SIMs are inserted, make sure your Call SIM is configured as your Data SIM. Open [Settings] >> tap [Mobile networks] >> make sure your Call SIM is configured as Data.