In a word:
vue2 uses vetur, and vue3 uses volar

Error indication:

vue2 items in the template must be wrapped by using a root element;

Solution:
in vue3 project, Vue related plug-ins use Volar

In the vue2 project, use the plug-in Vetur

1. disable Vetur in the current workspace. To prevent affecting the use of other projects

1. 1. E
   nable Volar

No more errors