If this is the section at the TOP of your page it is probably due to your theme. Many themes add spacing which is frustrating to troubleshoot. Have a look at this plugin: Fullwidth Page Templates. From the makers of the Astra theme, this takes any theme and allows you to control the header and footer.
If this is a section other than the Top of the page, make sure you are editing the section; then click on Advanced and make Padding and/or Margin adjustments as desired.
You can also open the page in a browser Inspector and see what is influencing that section.