I can't Open the New Office because the software requires macOS version 10.14.0 or later.

Upgrade your OS to the newest version that your Mac is capable of running.

For the 2017 MacBook Air that will be Monterey.

❗️ Do NOT upgrade until you have made a Time Machine or other backup of your startup drive.

Learn How to Back up your Mac with Time Machine

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

❗️ – IMPORTANT – Your 32-bit apps won’t work in Catalina, Big Sur or Monterey.

https://support.apple.com/en-us/HT208436

Use the free utility Go64 to check the compatibility of your apps:

https://www.stclairsoft.com/Go64/

✅ - IF your Mac and apps are compatible, then using Safari only, download the macOS upgrade installer app:

Click >  Mojave,  Catalina,  Big Sur  or  Monterey.

Once downloaded, double-click the “Install macOS [version]” app in the Applications folder to begin the upgrade.

Microsoft follows Apple's operating system support policy. The current released operating system (Monterey), and the previous two releases (Big Sur and Catalina). As of this post date, Microsoft still requires macOS Mojave for installation of Microsoft 365/Office 2021 for Mac. However, now that Monterey is released, expect Microsoft to very shortly set the minimum operating system to Catalina.