I tried to upgrade my Office 2011 to Office 365 but it said; This software requires macOS version 10.13.0 or later." Then I tried to upgrade to OS High Sierra and it said the same This software requires macOS version 10.13.0 or later.; " Any ideas?

I think you have an older computer that won't allow you to update your macOS. Here is the compatibility list: https://support.apple.com/en-us/HT208969

I just thought of something, do you have any antivirus, cleaning or any other third party "maintenance" apps installed?

What is the model ID of your Mac:

Look into the Free LibreOffice, it handles MS Office files…

https://www.libreoffice.org/

Apple macOS (Mac OS X)

The software and hardware prerequisites for installing on a Apple macOS computer are as follows:

• LibreOffice 6.3.x: macOS 10.10 (Yosemite) or higher
• LibreOffice 6.2.x: macOS 10.9 (Mavericks) or higher
• Intel processor
• 512 MB RAM
• Up to 800 MB available hard disk space
• 1024x768 graphic device with 256 colors (higher resolution recommended)

How did you attempt to upgrade to High Sierra and what version of Mac OS is currently installed on your 2011 iMac?

To upgrade from macOS 10.12 "Sierra" to macOS 10.13 "High Sierra" follow these instructions: How to upgrade to macOS High Sierra.