I try to change the colour of the background on a photo on an A4 sheet. But the background is not fill to the edge of the sheet on one side. the other three sides fill almost to the edge but one side there is a white section that cannot be filled in and it about 12mm or 1/2" wide.
What is the type of your printer and your OS? Can it print borderless?
If on Windows, make sure to first set the printer to borderless printing with Page setup (lower left) and then set borders to 0.
Also, there are some printers that have scale function in the bordeless mode.