Could you tell me please if these sentences are right?

I have two years experience.

I have a two years experience?

I have a two year experience.


a two year(s) experience is necessary.

two years of experience is needed.

two years experience is neded.
For me the correct phrases are:-

I have two years experience.
Two years experience is needed...........

The others are not natural for me.
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I'm sorry, but I must disagree.
Two years' experience
Two years of experience

But not
Two years experience
Hi guys

Another possibility is two-year experience. It's interesting to say that "Two weeks notice (the movie)" sounds acceptable to the guys at Hollywood. But then again they can hardly be considered grammar experts.
Two-year experience works in some situations.

I had a two-year experience I'll never forget working in the Peace Corps.

It doesn't work as "Two-year experience is required for this job."
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Thank you.