RCS message is not showing on iPhone 13

Your cellular carrier must support it. Check the document link provided below and look for your carrier. If it does not list RCS support then you will not see the option to turn it on.

Wireless carrier support and features for iPhone in the United States and Canada - Apple Support

My carrier is AT&T and my iPhone 13 already had RCS enabled and ready to go when I updated to iOS 18. I didn't have to do anything to turn it on. It’s not anywhere near universal yet, as much as Google was advertising it is.

Also know that every link in the chain between your iPhone 13 and your Android friend's phone must support RCS as well. Important to note that not all Android phones and not all carriers support RCS.

Do you have the option at Settings > Apps > Messages? If there is no RCS option there, then it is a clear indication that your carrier does not support it for your iPhone yet.