Steps to Reproduce:

Does this issue occur when all extensions are disabled?: No
having same issue.. :/
I saw it too. These days VSCode is used frequently, but I hope you can fix it with good cheer.
Same issue here.
Same for me, but seems to happen only on welcome page
Same issue here.
Same issue here, I disabled the extension for "IntelliSense for CSS class names" by Zignd and stopped receiving the message. For double checking, Re-enabled that extension and it appeard again. So something related to that extension possibly?
@mmosfetx is right, I disabled it and the error message disappeared :)
If you have further interest it looks like it can be tracked at the devs github page https://github.com/zignd/HTML-CSS-Class-Completion/issues/114
Thanks for @mmosfetx for pointing out the culprit. You can subscribe to https://github.com/zignd/HTML-CSS-Class-Completion/issues/114 for updates.
Meanwhile a tip: hovering over the "error" should now show the misbehaving extensions.
I have same issue here.
Same issue here
Same issue here
Most helpful comment
Same issue here, I disabled the extension for "IntelliSense for CSS class names" by Zignd and stopped receiving the message. For double checking, Re-enabled that extension and it appeard again. So something related to that extension possibly?