Hello catrina134,
This page can help provide assistance with deleting messages on the iPhone: Delete messages on iPhone - Apple Support
Delete messages on iPhone
In the Messages app
, you can delete messages and entire conversations from your device.
With Messages in iCloud, anything you delete from iPhone is also deleted from your other Apple devices where Messages in iCloud is turned on. See Use Messages in iCloud.
Note: Deleting messages changes only your own Messages conversations, not those of your recipients. To retract a message you sent accidentally, use undo send instead.
Delete a message
1. Touch and hold a message bubble, then tap More.
2. Select the message bubbles you want to delete, then tap
.
It may also help to try giving the device a quick refresh by performing a standard restart: Restart your iPhone - Apple Support
Be sure to also see if there are any iOS updates, as iOS 16.3 was recently released this week, and can help ensure the latest improvements are applied: Update your iPhone or iPad - Apple Support
Cheers.