The mode dial is the knurled knob on the top of the camera - if you didn't already know. This same error message displayed when the knob is between selection. If you have moved the know to a valid position - where it stops in a detent
, the message should clear from the display and be able to take pictures.
If moving the mode dial so that it rests in a detent and the message is still displayed, it indicates a problem with the switch inside the camera (that is operated when the mode dial is rotated). This is not a user serviceable part, and should be professionally repaired. I found United Camera
to list a Mode Dial repair to be $42. there are plenty of others that can make this repair for you, too - this was simply the first I saw that had a price (I have never done business with nor am I affiliated with United Camera in any way).
I hope this is a good starting point for you & good luck. Please rate my reply - thanks.