Office 365 installation on IO x EL Captail

Who is "they"? Microsoft 365 (was Office 365) is a "standard" type installation with no special conditions other than bit architecture compatibility.

What device are you trying to install it on? iOS is for iPhones.

Read here: https://support.microsoft.com/en-us/office/upgrade-macos-to-continue-receiving-microsoft-365-and-office-for-mac-updates-16b8414f-08ec-4b24-8c91-10a918f649f8

Apparently you need to upgrade to at least MacOS 10.15 Catalina.

What you posted was cryptic and vague.

You need to upgrade to MacOS 10.15 Catalina which has nothing to do with iOS. iOS is for iPhones, MacOS is for Macs.

I searched the web and quickly found this: https://support.apple.com/en-us/HT211683

What problems, and/or error messages, are you encountering?