I think I know. I am in the mobile version. You are on the desktop website on a Mac, right? Do you have an “AA” icon to the left of the address bar near the top of Safari? If you tap it, is there a choice to load the mobile website?
When you have loaded the mobile website, you shouldn’t have the text formatting toolbar along the top edge of text entry boxes. Then just paste the embed code in the text box and publish.
On an iPhone, you are in the mobile website by default so you should be able to paste the code directly.