Before we create your SeeMeQR, we sanity-check the URL that you provide by contacting that page. If we get any bad result, like a 404 'page not found' error, we issue a warning instead of making a SeeMeQR QRcode.
This is for your protection, so we don't provide you with a QR code that does not work.
You can override the protection by checking the 'Ignore 404 errors' checkbox. Proceed at your own risk.
The most likely reason for checking that box is that you want a QR code for a page that is planned but not yet publically available.