Custom ROMs almost never have Google apps and services built-in, partially to give users more choice, and partially to avoid any legal issues. If someone wants the Play Store and other Google apps, they have to flash an additional zip file containing those components. Open GApps has been the most popular source for these zip files for a while now, and it is now making builds for Android 8.1 Oreo.
The project’s site now has download links for Android 8.1 Gapps packages, in all platform variants (arm, arm64, x86, and x86_64). Most of the usual package options are already available, ranging from the ‘aroma’ graphical installer to the bare-minimum ‘pico’ build. If you’re getting ready to update to LineageOS 15.1 (or another Oreo-based ROM), OpenGapps now has you covered.