Problem with the bibunits

I have a problem with compiling bibunits. Even the most easy example (quite similar to the one published in the official documentation for this package) does not work:

documentclass[english,12pt]{article}
usepackage[utf8]{inputenc}
usepackage[T2A]{fontenc}
usepackage{bibunits}

begin{document}

begin{bibunit}[plain]
I want to cite two works, the first one~cite{Dub1} and the second one~cite{Dub2}. 
putbib[mybib]
end{bibunit}

begin{bibunit}[alpha]
I want to cite only work, that is~cite{Har}. 
putbib[mybib]
end{bibunit}

end{document}

There are 3 errors:

I found no citation commands---while reading file bibunittest1.aux
I found no bibdata command---while reading file bibunittest1.aux
I found no bibstyle command---while reading file bibunittest1.aux

Also, if I want to do something like that:

documentclass[english,12pt]{article}
usepackage[utf8]{inputenc}
usepackage[T2A]{fontenc}

begin{document}

I want to cite two works, the first one~cite{Dub1} and the second one~cite{Dub2}. 
I want to cite only work, that is~cite{Har}.

bibliographystyle{plain}
bibliography{mybib}

end{document}

so, without using the package, all is good. I will be very thankful for your help.