What version of Matlab are you using? For some reason that function's help page does not say when it was introduced as most do, though I know it was recent. It is possible that the version it was introduced it was not supported for C code generation, but has been made so in subsequent versions (my help for R2017a says it is supported). If you have an earlier version that could be the cause.
It is basically just a wrapper function for some simple functionality based on xcorr though, with a huge amount of validation and argument checking added in to make it look more complicated than it is! Find the lag giving the best correlation and move one or other signal by the appropriate amount. I haven't used alignsignals myself so I don't know what options it has for dealing with the ends of signals that are not matched and any other issues, but you can easily handle those for your own usage.
