Can messages deleted in error be restored?

If you have turned on iCloud Messages (Settings > your name > iCloud > Messages = On) then they are gone. If you did not and the messages you deleted are in your backup you can restore your iPhone from that backup. You will lose and data added since the date and time of that backup.

Delete a message or conversation

When you delete a message or conversation, you can't get it back. So make sure that you save important information from your messages.