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.