Although you see it as a line,Word actually thinks that it's created a border. So you need to remove that border:
Quote:
"To remove the line, place the cursor in the paragraph just above where the line was added. Make sure the “Home” tab is active. If not, click the “Home” tab on the ribbon.
In the “Paragraph” section of the “Home” tab, click the down arrow on the right side of the “Borders” button and select “No Border” from the drop-down menu. This removes the line from below the paragraph in which you placed the cursor."
Source (with pictures):
https://www.howtogeek.com/217924/how-to-remove-automatic-horizontal-lines-in-word/