It is not possible to capture screenshots on these types of pages:
Blank tabs with no URL (e.g. welcome browser tab)
Chrome Web Store webpages
Internal browser pages starting with this web address
How to fix?
Simply visit another valid URL and try capturing a new screenshot there.
If you are unable to capture a page type that is not listed above, make sure that your website allows external iframes to be loaded. If that's not the case, update your Content Security Policy to allow Marker.io to be loaded by following this guide
It still does not work!
Finally, if it still does not work, Make sure that your cookie settings allow Marker.io to work properly -> Check this article for more information.