I appreciate that you have included the image of your online store as that is super helpful! Could you provide the online store URL and let us know which theme you are using? As this context will help us determine the next proper troubleshooting steps to have this cleared up. If you are unsure about which theme is live on your store, you can navigate to the Theme Editor and select the three dots that are located in the top left of the page. I took a screenshot of my test store to show you the section I am referring to:
To solve the first issue you need to find the line {% section ‘sezzle-widget’ %} in your ‘templates/product.liquid’ template and remove it. Can you confirm that you have this line and when you remove it — the error message disappears.