Re-Install MacOS Sierra
You have to select AppleAPFS on the top, click Erase but if you try to run recovery option, it will install Mojave again. So to have Sierra, I would suggest the following steps:


First, connect your Macbook with an ethernet cable as it takes an hour to get full installation (basically depends upon the internet speed also). 
After this, turn off your computer and Reinstall from macOS Recovery mode with a combination of Shift-Option-⌘-R (this allows to install the macOS that came with your Mac, or the closest version still available.)


Basically, as per Apple Guidance:

Command (⌘)-R
Install the latest macOS that was installed on your Mac.

Option-⌘-R
Upgrade to the latest macOS that is compatible with your Mac.

Shift-Option-⌘-R
Install the macOS that came with your Mac, or the closest version still available.

Let me know how it goes.