Been getting this message a lot today; never seen it before and no idea what it means - any clues?
Are you running low on drive space? If you have Undo Control set to All, then AutoCAD is writing everything you do to a file, stored in the Temporary Drawing File Location, a setting you control through Options (Files tab.) On a default install, this is C:\Users\<<UserName>>\appdata\local\temp\.
One action that is likely to solve your issue is to close AutoCAD and then delete the contents of your Temp folder. Run AutoCAD again. Don't worry if the system can't delete some files, just bypass them.
I forgot to tell you: Be sure to turn Undo back on. Undo command, Control setting, All setting.
What that message means is that AutoCAD automatically set Undo to None, which means you have no Undo ability until you turn it back on.