If the recovery screen prompts you with a message saying "Signature verification failed. Install anyway?" , select . This occurs because GApps packages are signed by third-party developers, not the official LineageOS team. Step 6: Reboot to System
Installing GApps requires precise timing. Flashing it at the wrong point will cause Google Play Services to constantly crash. Google apps - LineageOS Wiki gapps for lineage os 191 link