I always geht this error, when I shut down my mobile phone in the evening and turn it on in the morning. FP tells me that the message wasn’t sent which is wrong since I get answers to them. But this most probably is another topic.
Can it be that you dragged the SMS widget to your home screen? If so, just press it for some seconds and then drag it onto the delete button which will appear on top of the screen.
The notifications of a message which has not been sent disappears if you open your SMS app and click on the person with the little “danger-icon”.