Then you have to wait for your carrier to activate it, or call them and ask if they support RCS. And see the post above yours from amp68.
And you should read this→What is the difference between iMessage, RCS, and SMS/MMS? - Apple Support
Noting especially:
If you aren’t using iMessage, you can use RCS. RCS text messages can be sent to non-Apple devices as well as another iPhone or another Apple device with Text Message Forwarding turned on. With RCS, you can send texts, high resolution photos and videos, links, and more. RCS also supports delivery and read receipts and typing indicators. RCS messages appear in green text bubbles on your device.
Apple’s implementation of RCS is based on the industry’s standard. RCS messages aren’t end-to-end encrypted, which means they're not protected from a third-party reading them while they're sent between devices.
RCS is a carrier-provided service. When your device connects to your cellular network, it communicates with your carrier and their partners to set up RCS. User identifiers are exchanged for your carrier and their partners to authenticate your device and provide a connection. These identifiers could include but are not limited to your IMEI, IMSI, current IP address, and phone number. Your current IP address might also be shared with other RCS users.
To use RCS, you need iOS 18 and a text-messaging plan from a carrier that supports RCS on iPhone. Contact your wireless carrier for more information.
To turn RCS on or off, go to Settings > Apps > Messages > RCS Messaging.
Learn whether your carrier supports RCS messaging on iPhone