Note that the source of the problem is that there was an update to sig-alternate in May 2015.
Before May 2015, the copyright text had to be inserted manually into the document. With the recent update, new commands like
\setcopyright allow to conveniently choose between the texts for copyright-transfer, publishing-license, or open-access. In the last step of the ACM publishing process you will now be given a LaTeX code fragment which uses the new syntax. The most likely cause of the error is thus that you are trying to use such a new code fragment, but you haven't updated to the most recent template yet.
The solution is what @Sumit suggested: Simply download the most recent
acmcopyright.sty from the official ACM site.