I cant tell you what a pleasure its been / Its been a pleasure

I cant tell you what a pleasure its been is a way of saying what a pleasure its been. In other words, you are telling this person how incredibly nice it was to do this thing with them, be in this situation, have this reward or task, or to meet them. It is often used when you first meet someone or spend time with someone, and tell them what a pleasure it was to be in their company.