Sorry about the trouble you are facing. I would like to know if you were able to locate the "save to DXF" as suggested by @kevin stohlmeyer? If not, would you mind sharing a few more details like:
Did this start happening after upgrading to 26.4.1?
What is the Windows 10 build no.?
What is the canvas/ artboard resolution of the file you are trying to export as DXF.? Does reducing the artboard size help you get the export option?
Are there any recent changes made to your system? For example, any antivirus, system updates, or web extensions installed recently.
Does resetting Illustrator Preferences (https://helpx.adobe.com/in/illustrator/using/setting-preferences.html) help to resolve the issue? Disclaimer: Please note that resetting preferences will remove all the custom settings, and Illustrator will launch with default settings. You can also take a backup of the folders in case you want to. Location is mentioned in the article.