Have iMac and I couldn't install Microsoft office as it requires Mac OS version 10.13 or later

microsoft only support the 3 latests macOS version with their office 2019 and office 365

you can

1 purchase microsoft office 2016

2 upgrade your macOS to something newer than sierra macOS - How to Upgrade - Apple

3 use libra office which is a free office which open ms office files

Microsoft has already announced that they will be ending support of Office 2016 for Mac in October of this year (2020). Therefore, not a strategic purchase. And, since it is still a currently supported MS product, it too may fall into that category of requiring High Sierra or later for a new installation, or applying updates.

The LibreOffice Suite is a great clone of MS Office, but like any other clone software, there are LibreOffice features that when used, render documents indigestible, or change their appearance, when shared with others using MS Office applications.

If you are regularly exchanging Microsoft document formats with those using Office on Windows and Mac, then I would recommend either a Office 365 subscription, or the single-purchase Office 2019 for Mac.

Otherwise, if you are not sharing documents, then LibreOffice would be ok, and free has merit. Don't overlook the LibreOffice documentation, all created in LibreOffice Writer. Expect a learning curve, and experiment with the ribbon bar view that suits you in View menu : User Interface > .