This issue might occur if you don't add references for your `Jquery` , `SPService` , So make sure that you have the following references in your script. 

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.0/jquery.min.js"></script>
        
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery.SPServices/2014.02/jquery.SPServices-2014.02.min.js"></script>
    
    
    <script language="javascript" type="text/javascript">
        $(document).ready(function() {
            $().SPServices.SPAutocomplete({    
                sourceList: "Country",
                sourceColumn: "Title",
                columnName: "FilterTextBox",
                ignoreCase: true,
                numChars: 3,
                slideDownSpeed: 1000,
                debug: true
            });
        });
    </script>

Also, Make sure that The `sourceColumn` name is internalName for the column by going to List Settings > Edit Column > check the URL `Field=` parameter. but in your case, it's `Title` field that its internal name also `Title`

    /_layouts/15/FldEdit.aspx?List=%7B53AE6414%2D62F1%2D4FF8%2D979A%2D86F80090776B%7D&Field=Title




- For more details check [$().SPServices.SPAutocomplete][2]

- For SharePoint 2013 check [SPServices Stories #17: Multiple Form Fields Autocomplete for SharePoint 2010/2013 using JavaScript][1] 


  [1]: http://sympmarc.com/tag/spautocomplete/
  [2]: https://spservices.codeplex.com/wikipage?title=$().SPServices.SPAutocomplete&referringTitle=Documentation