Licking the nose constantly can be a symptom indicating seizure or can indicate nausea. If there is sneezing then it might be an allergy condition or a foreign body in the nasal passage.
Usually we see licking everything with seizure activity and not just the nose. Dogs will lick the air or walls, etc. You can read about seizures here:
Since nausea can definitely cause a dog to lick their nose repeatedly, you may want to try pepcid or Zantac. Read about Pepcid dosages and usage information here: http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
Zantac can be given to a dog at .25 to 1mg per pound every 8-12 hours. Read about usage here:
The nausea could be from many different things including an empty stomach, eating an unusual treat, poisoning, or excess bile to name a few.
If there is nasal discharge from a respiratory problem such as an allergy, this can cause a dog to lick the nose continually as well. If this is the case, Benadryl can be given to your dog. The dose is up to 2mg per pound every 8 hours.
You can read about these here.
Many times when a dog is licking their nose and sneezing, it is due to a foreign object in the nose such as a grass seed, leaf or even fuzz. So gently look up the nose and see if there is anything there. If you find something, do not attempt to remove it as the nasal passages are fragile and any item should be removed by your vet.
I hope that this information is helpful to you.