(Google Apps). GApps is a collection of proprietary Google applications and frameworks that are not included in the base source code of Android but are essential for a modern mobile experience. The Necessity of GApps
Your device bootloader must be unlocked. gapps android 51
The cleaner the installation, the fewer errors you will encounter. Always flash GApps immediately after flashing your custom ROM, before booting into the system for the first time. Method: Clean Installation via TWRP Recovery (Google Apps)