You flashed ARM GApps on an ARM64 device (or vice versa). Fix: Re-download the correct platform. Use a hardware info app like Droid Info to confirm your CPU.
Your /system partition is too small for the GApps variant. Fix: Use the pico variant. Alternatively, repartition your device (advanced, risky) or remove unused system apps via the .gapps-config file.
Open GApps is a community-driven project that provides Google Apps packages for custom Android ROMs. Since most custom ROMs cannot legally include Google services like the Play Store or Gmail, users must flash these packages separately. Open GApps is the industry standard due to its automation, reliability, and variety of package sizes. Why Android 7.1.2 (Nougat)?