That was the first problem. It is perhaps not enough documented, but your plugins must be in a folder with name beginning with peertube-plugin-. (and same for themes, where the name should begin with peertube-theme).
Then, check your package.json: have you renamed the plugin? from peertube-plugin-quickstart to peertube-plugin-updatevideo? The name must be the same as the folder name.
Maybe the error is due to previous installation. Try to check this file on your server: data/plugins/package.json (replace data with the complete path to your peertube storage dir).
Perhaps this file contains old plugin names. It should look like this:
{
"dependencies": {
"peertube-plugin-livechat": "file:/peertube-plugin-livechat"
}
}
If there are previous plugin references (something like "updatevideo:": "file://peertube-plugins/updatevideo"), just try to delete them.