Sorry, I am unable to validate this document because its content type is text/plain,

Hello, I apologize for my incompetence on the subject but I need advice on an error on my website. Thanks, the error is this: Sorry! This document cannot be checked.

Sorry, I am unable to validate this document because its content type is text/plain, which is not currently supported by this service.
The Content-Type header is sent by your web server (or web browser if you use the file upload interface) and depends on its configuration. Commonly, web servers will have a mapping of filename extensions (such as ".html") to MIME Content-Type values (such as text/html).
That you received this message can mean that your server is not configured correctly, that your file does not have the correct filename extension, or that you are attempting to validate a file type that we do not support yet. In the latter case you should let us know that you need us to support that content type (please include all relevant details, including the URL to the standards document defining the content type) using the instructions on the Feedback Page.
how can i fix it? Thanks again

Using apache? Your server is probably missing the mime module/has it disabled/incorrectly configured in the settings. Contact your server support and tell them to fix it.

Try reading these, hopefully it'll help:
https://www.coffeecup.com/forums/html-editor/problem-with-w3c-validation/https://forums.macrumors.com/threads/help-html-validation.106199/
If you still can't figure it out, you should submit a support ticket to your website hosting.