Normally we would say "It has been nice working with you."
Student or Learner
It has been a nice working experience with you.
I would like to know why we use "a" before nice and why without "a" sentence is not correct.
I agree that Mike's version is much more natural. If you want to use "experience", you could say something like "Working with you was a nice experience". Can you see why the indefinite article is required?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.