A family of Microsoft word processing software products for creating web, email, and print documents.
Your screen shot, part of which I am repeating below, indicates that you are running Word on the web, that is, the web browser version:
Word on the web is limited in many ways. For one thing, it requires that you use the most recent file format for editing.
If you are paying for a 365 subscription, install the local versions of Office applications on your computer. See https://support.microsoft.com/en-us/office/download-and-install-or-reinstall-microsoft-365-or-office-2021-on-a-pc-or-mac-4414eaaf-0478-48be-9c42-23adc4716658.