Print sheet on Ajax client is not supported. You must use IE plug-in for that functionality. If Ajax client is the default client on your company, the only workaround is using the browser print options. You can read more at the help are on developer, this is what describes about it:
" Print Sheet: Prints the sheet specified by Sheet ID. (Not available as document and sheet trigger.) This function does not work in AJAX client. "
Giuseppe Novello Principal Technical Support Engineer @ Qlik