[Support request] Trying to Add an HTML page into my WP website

The page of concern is https://kimn.ca/marksold

I am trying to add an HTML page. When we copy and paste the HTML code into the HTML tab of the page, the only thing that shows up is the header. We believe that the HTML is adding extra tags and removing others. Example – my son-in-law (who created the HTML page says):

My code says:
<p role=“alert” id=“errorMsg”></p>

Your code says:

<p role=“alert” id=“messageMsg”>

So, it is missing the closing </p> tag.

Then, just before the <script> tag, it has inserted a <p> tag….

So, somehow you have to get WordPress to honour the text you are entering verbatim instead of trying to style it for you.

Also, the <style</style> tags seem to be missing, so the page won’t look right either.

So I thought the problem might be the TinyMCE plugin. I deactivated it but that didn’t change anything. So then I thought I could bypass the WP interference by adding the code directly into PHPMyAdmin, but again, nothing changed. The only thing we see is the header.

Can you tell me please how to “get WordPress to honour the text you are entering verbatim instead of trying to style it for you.”

Many thanks!