This warning occurred because the printer is using a different paper size than the one you selected.
Why did I get a blank print?
If the app had tried to print anyway, then your print could have been wrong, wasting your ink and paper. Instead you can reuse your blank sheet of paper and try again.
What can I do to fix this issue?
Choose a different paper size. If you tap on “Choose a Supported Paper Size”, the app will show you the paper sizes that are currently available in your printer.
If a valid paper size is being rejected, then there could be a problem in the paper size detection system. Please try turning off your printer and your iPhone or iPad, then turning them back on. It may help to open and close your paper trays or to refill them, in case your printer is failing to detect them.