Technical documentation HTML project

I’ve been trying to duplicate the look and feel of the pages in these projects more or less exactly. What can I say, it’s just the perfectionist in me. But when I tried inserting the menu at the left as an iframe (that’s my guess as to how it’s done, anyways), with the following code from, as follows:

I got the following error message:

Since it doesn’t look like it’s gonna take that, what am I supposed to do to duplicate the look and feel of the page? Or, should I just worry about the menu on the left (or top on my phone) and not worry about duplicating the content on the right, or…?

I assume you are on about this project:

I have no ideas what that /boomboom/ file path is. Have you activated a plugin on Codepen that could be causing the problem?

No iframe. Use a <nav> element, as shown in the instructions.
Please post your actual code and or link to your Codepen page.