By default, you start with a one column backend layout; only the normal column is visible. In older versions of TYPO3, there were 4 columns: left, normal, right, border. When you click on Page and then on the home page, you'll see the column(s):
You need to make sure that you can add content in the backend for the content_main part and for that part only. So you'll need to create a custom backend layout. To do this, click on list, click on the "storage folder" and click on the green plus at the top of the content window to add a new content element and choose "backend layout":
You'll name this backend layout "Main 1 column with menu" and put the following code in the "Config" box:
colCount = 1
rowCount = 1
name = Content main (with menu)
colPos = 0
You'll end up with:
Please note: for recent versions of TYPO3 (from v10 and up), you can only enter the column name and the column number by clicking the pencil in the config window. In this example, the name will be Content main (with menu) and the column number will be 0.
Save by clicking one of the save icons.