I can't install Office 365 in my iMac macOS 10.13.6

your options are

1. update your mac to a version that office require if the macs hardware support that
2. if it do not get a newer mac which are supported my microsoft
3. use an alternative office suit like libera office or even apples iworks

Contact Microsoft for support on their product.

If you are using the online version, you might be able to use Firefox or Chrome to access office 365. Non-Apple browsers are more up to date on older computers.

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/