1、 Resolution:

Literal meaning: Vue is not defined, that is, the introduction of Vue source code;

2、 Try to resolve:

1: Try 1: check whether the Vue source code import (path and script tag) is wrong
2: try 2: there is no problem with the import syntax, that is, check whether there is a Vue source code file in the target folder, and find no (reason: I added the Vue source code file manually later, but it was not generated in time in the target)

3、 Solution steps:

Click clean to clean and regenerate the target file. The Vue source code is successfully introduced and the error is resolved;

Screenshot: