To run Sierra 10.12.6 Beta 5, your Mac must be from this list (minimum):
In July 2017, Apple released macOS Sierra 10.12.6 Beta 5 (Build 16G23a) to developers and public testers. This build represented the final stabilization phase of the Sierra lifecycle before the launch of macOS High Sierra (10.13). Macos Sierra 10 12 6 Beta 5 Dmg Xcode Beta Dmg
According to multiple sources, discovered during the beta testing process of macOS Sierra 10.12.6. Beta 5 was essentially the final code, and the public release that followed a few weeks later (July 19, 2017) only added security patches and minor enterprise fixes. A German tech site even speculated that beta 5 might have been the Golden Master (GM) build. Therefore, if you install the public DMG from Apple’s CDN, you are getting the same code that developers tested as beta 5. To run Sierra 10