Hi @Yanhong Li
then it must depend on your subscription and/or maybe on your quota.
For example I can (at least) see those kind of machines in my MCT subscription for West Germany Central
What kind of subscription do you have? Student, MSDN, other kind of sponsored, company order your own PAYG subscription?
In some sponsored subscription it is not allowed to deploy N-series, in a company sub it might be also forbidden by policy to select those kind of machine.
Maybe you haven't searched or regions with a "wrong" filter or it is "not allowed" or blocked.