Watch this guide for a visual walk-through on how to correctly flash GApps to your device:

After sideloading the LineageOS ROM, you often need to reboot back into recovery mode before sideloading GApps to ensure the system partitions are correctly mounted.

: Often cited as a reliable alternative if others fail to flash or work correctly on specific devices. Installation Guide For a successful installation, GApps be flashed before the first boot of the OS.