Most custom ROM users on F21 Pro avoid GApps entirely. Instead, they use:
The glow of the monitor was the only light in the small, cluttered workshop. Elias squinted at the screen, his eyes red-rimmed from hours of staring at code. He wasn't looking for treasure, or state secrets. He was looking for a signal.
Set the tool to "Download Only" or "Firmware Upgrade," click download, and connect your powered-off phone.
ROM available at: github.com/qinux-team/qin_f21_pro_releases
Enter the ROM community. Here is my experience turning this spyphone into a functional daily driver.
Usually caused by not wiping data after flashing.