Reinstall macOS after factory resetting my 2015 MacBook Air

Are you booted into Recovery or Internet Recovery Mode or from a USB installer? If so, then you may need to launch the Terminal app from the Utilities menu and force unmount all the volumes on the internal SSD. In theory the internal SSD should be "disk0" (you can confirm by checking Disk Utility) so the following command should unmount all volumes on the internal SSD:

diskutil  unmountDisk  force  disk0

If this command completes successfully, then quit the Terminal app and launch Disk Utility to erase the whole physical SSD before installing macOS. I had to do this recently because even in Recovery Mode macOS would not unmount the disk normally.

If you are attempting to install macOS 10.12 Sierra, then you need to erase the entire physical SSD. You have highlighted the physical SSD in your second screenshot. Erase the physical SSD as GUID partition and MacOS Extended (Journaled). Once properly erased you should be able to select "Install macOS".

If Internet Recovery Mode gives you problems, then create a bootable macOS USB installer using another Mac. Or connect the MBAir 2015 directly to the router using an Ethernet Cable & a Thunderbolt to Gigabit Ethernet Adapter.

Also here are all the steps you should have performed before erasing the SSD whenever you are selling an Apple computer:

https://support.apple.com/en-us/HT201065

Edit: Even though macOS 10.12 Sierra is able to recognize an APFS volume it is only very basic support. Sierra cannot be installed to an APFS volume. Even if it could be installed to an APFS volume you should still erase the whole physical drive.