Not all printers can print a paper up to its edges. Most of them leave an unprinted margin.
LibreOffice offers a semi-automatic feature that enables you to print as close to the paper's edge as is possible.
Make sure that your printer has been setup under File - Printer Settings.
Make sure that the Web in the View menu is not selected.
Select the Format - Page command, and go to the Page tab.
Under Margins you can define the maximum or minimum possible value for the page margins (left, right, top, and bottom). Click into the respective control, then press the Page Up or Page Down key. The preview displays a dashed line around the printable range.
Click OK to close the dialog.