Error on custom email plugin

I have written a simple email plugin that will take the html between two tags and send it in an email. The problem is that it works on my local machine but on the production server I am getting this error

Notice

Undefined index: email_template

ee/legacy/libraries/Extensions.php, line 176 hide details

Severity: E_NOTICE Error

Unable to load the following extension file:

ext.email_template.php

Any ideas?