The word "disappoint" is used when a person is unhappy with a situation. In this case, there was an expectation for a positive outcome or a belief that something good would happen, but instead the opposite occurred.
You really disappoint me.
I’m very disappointed in you.
Many Americans are very disappointed by the results of the election.
Democrats are disappointed that so many Republicans supported a nominee with a pessimistic view of America.
It’s very disappointing that a well-qualified candidate was beaten by someone who has no experience as an elected official and has very little knowledge of government or the U.S. Constitution.
The word "disappointment" is a noun:
It’s a huge disappointment to learn that a large number of Americans supported a candidate who has a history of making unwelcome advances upon women.
Disappointment with the election results will show in the faces of those who believe a great opportunity has been lost, and a huge mistake has been made.