Are you trying to upgrade your Mac from Sierra to High Sierra, the full installer of High Sierra can only be downloaded via the App Store, follow the link here,
After testing and experimentation of several beta versions, now Apple has released the macOS 10.13.3 Final Version (Build 17D47 / 17D2047) for macOS High Sierra powered Mac devices. The latest macOS 10.13.3 Update is instantly available for upgradation. High Sierra is an update of MacOS which focuses mostly on back-end developments and a higher level of user security. As such, this operating system lays a foundation for future upgrades. MacOS High Sierra for Mac. CyberLink PowerDirector. Advanced features for spotless results. Office Home & Student. Mac OS X High Sierra 10.13 by Apple, Inc. Publication date 2017-09-25 Topics. DOWNLOAD OPTIONS download 1 file. ISO IMAGE download. Download 1 file. ITEM TILE download. Download 1 file. Download 1 file. TORRENT download. سیستم عامل جدید macOS High Sierra ، اپل هر سال در کنفرانس WWDC نسخه ای اولیه از سیستم عامل های جدید Mac و iOS را عرضه میکند و در شب گذشته هم این کمپانی از iOS 11 و سیستم عامل کامپیوتر اپل با نام جدید macOS 10.13 High Sierra رونمایی کرد.
https://support.apple.com/en-us/HT208969, scroll down to part 4 where there is a further link that will take you to the App Store where you can download the OS. This will install the latest/ last version of High Sierra 10.13.6
Download Macos Sierra 10.12.1 Update
It sounds like you were trying to install an update to High Sierra, possibly the Combo 10.13.6 from the Apple Support Downloads page, you must have an existing copy of High Sierra to do that as that is termed an update. Going from one OS to another, newer OS is called an upgrade and can only be done through the App Store.