Unfortunately this could be the start of a long and expensive saga.
The dealer will most likely propose a new AdBlue level sensor - which only comes with the whole AdBlue tank - and cost around a grand.
You'll be better off finding an independent specialist who might be able to clean the sensor - sometimes the AdBlue crystallises - but even if you do need a new tank, the specialist will be much cheaper than the dealer.
What you can't do is ignore the problem... because at some point it will start telling you that you have only 500 miles before the car won't let you start the engine again.
EDIT: How old is the car? When was it bought? Is there any warranty?