Failed to Activate multi-task selection

I want to activate multi-task selection,but something happened when I follow the guide-

my program based on vue

i only add below content on the index.html

script src="/node_modules/dhtmlx-gantt/codebase/dhtmlxgantt.js">
script src="/node_modules/dhtmlx-gantt/codebase/ext/dhtmlxgantt_multiselect.js">
link href="/node_modules/dhtmlx-gantt/codebase/dhtmlxgantt.css" rel=“stylesheet”>

but it seems can not activate multi-task selection
could you tell me what should i do?

Unfortunately, I couldn’t reproduce that issue.
Here is a demo project with the multiselect extension:

Thank you very much! I upadate my package.json,the problem is solved.