This happen because in your script you provide only the url of the action but don't provide any value for [ss].
You must use another syntax that allow providing [ss] value in remote call, for example using a function as source for autocomplete and an explicit JSON call to remote server:
source: function( request, response ) {
$.getJSON( "/Home/AutocompleteSuggestions", {
ss: request.term
}, response );
},
You can find more info on JQuery documentation about
autocomplete and
getJSON.