Should I cite an article that I can't find? I am facing a problem, the article seems a very important reference that everyone talking about the subjects is citing it. However, looking at these articles, they can't give a reference or link to the cited article. Even I use the volume number, article number to search on the net, I still can't find it.

For example, this article, only the second reference doesn't have a pdf link pointing to it. I think the author may be like me can't find the article, but sees others citing it in a very important position, so he also cites it. I can't exclude the possibility that the author reads a paper-based version. 

When you search the citing article, many of them can't give a link to that paper, and I can't find it either. Should I cite it anyway? Because from the inspection, this paper seems an indispensable reference when talking about the subject. On the other hand, I can't find it (even on the official site) and I didn't read the abstract or even the title. I think this is not quite equal to the case where the article is behind the paywall, because behind the paywall you at least see the title and abstract.
I would definitely try contacting your librarian at your university library. University libraries collect physical copies of papers and they might have this one. Librarians are also a lot more versed in navigating the various search systems and they will probably be able to find this paper, if only as a hard copy. It is also possible that your university does not have the paper, but they could contact different universities to get it.

I would definitely recommend to no cite what you haven't read, especially if you don't even know the title of the paper.

Edit: As others have also noted, it might be good practice in your field to in fact cite the paper, but with the caveat that you cannot find it, as suggested by Thredolsen's answer. If this is common in your field please do this.