you switch to app.vue import and try

are the / theme/index.less files you created and main.js in the same file directory?
look at the error report. Is your file directory wrong?

follow the steps in the ide/theme-sharpTGAZGJXG" rel=" nofollow noreferrer "> document : install npm globally or locally in the project? Try

is it vue-cli3?

if yes, you need to install this plug-in vue-cli-plugin-iview