A family of Microsoft word processing software products for creating web, email, and print documents.
I'll say up front that in desktop Word from Microsoft 365, version 2607 (Build 20228.20110) Current Channel on Windows 11 Pro, the File Conversion dialog (the panel you mentioned) does appear when choosing Save As to Plain Text.
To understand what's happening on your system, we'll need more information about your setup:
- Are you using desktop Word or Word on the Web?
- If it's the desktop, what are the version and build numbers shown at File > Account > About Word?
- Is it on Windows, Mac, or another operating system?
Try some basic troubleshooting steps:
- Does the Save As fail with every document you try, or just with one or some but not others?
- If the latter, is there something in documents that fail that isn't in the others?
- Use the troubleshooting steps in https://learn.microsoft.com/en-US/office/troubleshoot/word/issues-when-start-or-use-word -- but leave the registry changes (the third and fourth sections) until after you've tried all the other steps without success.