why can't i use word after the most recent update ?

Older versions of Word aren't compatible with macOS Catalina.

If you have an early version of Office for Mac 2016, you can update it at no charge. However note that connecting to Microsoft 365 services using Office 2016 for Mac is not supported by Microsoft after October 13, 2020. See: https://docs.microsoft.com/en-us/office365/troubleshoot/administration/office-2016-for-mac-64-bit-upgrade

If you have Office for Mac 2011 or earlier, you can either subscribe to Microsoft 365 or you can make a one-time purchase. See: https://products.office.com/en-us/buy/compare-microsoft-office-products-for-mac?ms.officeurl=mac-compare-redir

Or use the free Microsoft Office on the web: https://products.office.com/en-us/free-office-online-for-the-web

Or Google Docs: https://www.google.com/docs/about/

Other options include using the free and very good Libre Office https://www.libreoffice.org/

or Apple's Pages and Numbers

Microsoft (like most other software companies) does not offer unlimited free updates for their product (unless you pay for a subscription to Microsoft 365).

There is no way to tell what version you have based on the serial number of your Mac. You may be able to guess by knowing approximately when you bought it.

You can open your old documents using a newer version of Office or using any of the other free products I listed in my first response. Microsoft Office on the web will likely be most compatible but the free LibreOffice is very good and very compatible.

Your Office documents are independent of the 32-bit/64-bit application issue, and will open just fine in the 64-bit version of Office 2016 for Mac (retired Oct/2020), Office 2019 for Mac, Microsoft 365 (subscription), and the free, LibreOffice Suite.

Microsoft retired Office 2011 for Mac in Oct 2017, so if that was your single purchase, it is no longer in effect for new Microsoft versions that I mentioned above that run on Catalina. Software has a shelf life, and new versions cost money.

You can no longer reinstall Office 2011 for Mac on earlier versions of macOS because Microsoft has deactivated those license servers.

I have asked the hosts to remove your serial number as that is private information, and we are not Apple Support.

I too am sure that you likely have an older non-compatible version of Word installed. If you wish to see what version it is, you can open your Applications folder, single click on the Word icon and then either right click or use the Get Info tab in the File menu. This will open a window which will display the information you want.

Since you didn't provide any information of which exact version of Microsoft Word that you using, I can only surmise that it is a pre-2016 or a 32-bit version which won't run on macOS Catalina.