Why didn't this vue custom component be rendered?

I wrote it according to the tutorials on vue"s official website. There are two files altogether:
main.js

</script></body>
</html>

but there"s nothing on the page, and the F12 check element can"t see this custom component either, only in the body tag

has a line of this,
what"s the problem? Ask the master for advice

Feb.25,2022

I ran your code, (excluding the sentence import App from'. / app.vue';, because I don't have an app.vue file locally.
everything is fine, consistent with the official website

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e4ba19-44e50.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e4ba19-44e50.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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