My name is Lilith, I'm on the Social Care team at Shopify.
If you're seeing an error message related to an HTML issue, this is coming straight from your theme code that would have been modified.
When themes are customized or edited in the code, anyone that was working on that for you should be able to correct that. Otherwise, if you're not too familiar with HTML/CSS/Liquid, then you can revert the theme code back to the original theme default code.
This will ensure that your page will at least be functional until you can get a hold of your developer to correct anything they added. We've got a step by step with images guide in our theme developer pages to help you with this reset.
For the dynamic checkout button. Are there specific buttons that are missing?
When you use a mobile device you'll be able to see the appropriate buttons on either Chrome or Safari. If you're not on a mobile device you won't see them since desktops/laptops (not related to Shopify) don't have those easy pay features.
Our help center has the full guide on how to set the buttons up as well as a guide on how to test whether your buttons will work on different devices and browsers. If you don't have Google Pay or Apple Pay for example on your mobile devices, then that guide will help you test them on your laptop browser.
Let me know how you do with the information above! I'm happy to answer any questions you think of as you work on your business.