Next, you need to tell TYPO3 that it needs to use the "one content column with side menu" content template (main_1_column_with_menu.html) to show the content. You'll have to edit the main typoscript template and add the following:
page.10 = FLUIDTEMPLATE
format = html
file = fileadmin/templates/main_1_column_with_menu.html
content_main < styles.content.get
content_main.select.where = colPos = 0
As shown here:
Template "format" is html by default. Should you want to use template files with another extension (such as htm or txt), you'll also need to change that here. With "file" you indicate which template file you want to use. The variables indicate which parts need to be filled with content (content_main) and with which part in the backend layout it corresponds (colPos = 0).
Save the typoscript template. To make sure all changes will be effective, clear all cache by clicking the lightning bolt on the top right of the TYPO3 window.