my iMac (21.5, 2017) is running macOS Sierra v 10.12.6. I can't upgrade into 10.13 version

Depending on your Mac, it might be possible to upgrade to Mojave or Catalina.

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Check compatibility

If a macOS installer can't be used on your Mac, the installer will let you know. For example, it might say that your Mac doesn't have enough free storage space for the installation, or that the installer is too old to be opened on this version of macOS.

If you want to check compatibility before downloading the installer, learn about the minimum requirements for macOS Catalina, Mojave, High Sierra, Sierra, El Capitan, or Yosemite. You can also check compatible operating systems on the product-ID page for MacBook Pro, MacBook Air, MacBook, iMac, Mac mini, or Mac Pro.

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Download macOS

It takes time to download and install macOS, so make sure that you're plugged into AC power and have a reliable internet connection.

These installers from the App Store open automatically after you download them:

• macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Mojave 10.14 can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion
• macOS High Sierra 10.13 can upgrade Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion, Lion

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copied from: How to get old versions of macOS - Apple Support