Your system partition is too small to accommodate the GApps variant you chose (common with Full or Stock packages on older devices).
Android 6.0.1 was a pivotal update in Android history. It introduced:
Android 6.0.1 devices generally fall into one of three architecture categories: gapps android 6.0.1
For newer 64-bit processors (e.g., Snapdragon 808/810/820 and later).
Designed for newer 64-bit processors (e.g., Snapdragon 410, 615, 808, 810 and newer). Your system partition is too small to accommodate
Re-verify your phone’s CPU specs, download the matching GApps for Android 6.0, and re-flash.
GApps on Android 6.0.1 unlock specific OS features that were revolutionary at launch: The Open GApps Project Designed for newer 64-bit processors (e
(Lightweight & Stable)
represents a critical bridge between a "de-Googled" experience and the modern convenience of the Play Store. Because Android 6.0.1 was a "refinement" era for Google, these GApps packages are legendary for their stability and efficiency. The Role of GApps on Marshmallow