reply to a specific message missing in my iphone 12 mini

Is this an iMessage or SMS? If it is an iMessage, when you double-tap to get Tapback, you should not get the Copy Move that you show. If this is an SMS, you cannot send an individual reply.

Send as SMS is a setting that only comes into play if your iMessage is not available. SMS is what you send to those people using an Android device.

Again, the screen photo that you displayed is how the screen would look if you long pressed on a message to bring up the Copy and Move commands. The Reply for an individual message appears when you double-tap a message to bring up the Tapback and reply.

See the difference?