Depending upon the type of printer, this could be one of a few things:
The Receipt Paper Was Installed Improperly
To fix this, simply turn the roll 180 degrees so that the other side of the paper is hanging out of the printer (left side should now be on the right).
The Wrong Receipt Paper is Being Used
Thermal printers will need thermal receipt paper (It reacts to heat. Test it near a heat source to see if it darkens)
Impact printers will need impact receipt paper (it reacts to pressure. Test it by
scratching a finger nail against it).
The Ribbon Could be Installed Improperly or It Could be Out of Print Ink on the Ribbon
(This applies for Impact printers.)