Upgrade a partially Downgraded Moto G4 Plus (XT1644) with unlocked bootloader to Nougat
You can update to Nougat by downloading stock firmware and flashing it via fastboot. For full guide go to this xda thread.
Method and files will work for XT1640/XT1641/XT1642/XT1643/XT1644.

fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash dsp adspso.bin
fastboot flash oem oem.img
fastboot flash system system.img_sparsechunk.0
fastboot flash system system.img_sparsechunk.1
fastboot flash system system.img_sparsechunk.2
fastboot flash system system.img_sparsechunk.3
fastboot flash system system.img_sparsechunk.4
fastboot flash system system.img_sparsechunk.5
fastboot flash system system.img_sparsechunk.6
fastboot flash system system.img_sparsechunk.7
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
fastboot erase cache
fastboot erase userdata
fastboot erase customize (optional: will erase any customizations if ordered through motomaker)
fastboot erase clogo (optional: will erase the custom boot logo if ordered through motomaker)
fastboot oem lock
fastboot reboot

As you stated in the question that you are unable to run fastboot oem lock begin so skip this command and you will update to android N with an unlocked bootloader.