I reproduced the issue when the 4.0 version doesn’t work in a project where the 6.0 version works correctly. Unfortunately, I don’t specialize in vue.js, so I cannot tell you if you need to apply the additional code in vue.js to make the 4.0 version work. However, the same project works correctly with the 6.0 and 6.1 versions.
Here are the demo projects with Gantt where everything works correctly:
Unfortunately, I cannot reproduce your use case because you sent only 1 file. If you cannot send the project with the source code, please, reproduce the issue in a test project and try adding Gantt there, then send the project to me. I will reproduce the issue locally and check if there is a way to start Gantt.