Hi,
probably your localhost GeoServer is not reachable from the MapStore backend.
In fact, while most of the webmapping operation ( like pan and zoom, feature info etc...) are performed by your browser, the print operation are performed by MapStore server.
So in that case, all the resources that you want to print must be reachable from the MapStore host machine (typically public on internet, on in the local network) with the address you specified.
"localhost" is a reserved address that means "this machine". It works when "this machine" is the same of the browser for web browsing, but you must specify a valid address, when you want to print, and make you sure that the address and the GeoSerer is accessible from the MapStore host machine.