You just made a paper change. The point-of-sale system is instructed to print a customer's receipt. A sheet of paper that looks to be the correct size for a customer's receipt is printed by the printer, but it is entirely blank.
Make sure the roll of paper you just loaded into the printer is a thermal receipt paper roll. If it is, then take the paper roll out, flip it over, load it back into the printer.
Or you can refer to Step put thermal paper roll in printer here