I'll assume you mean that you do not see a From field in the header when you compose a new message in the editor. Not sure about the 'cannot be sent' part though.

Anyway, if so, see "View" in the editor's menu, or just right-click on one of the other header fields such as 'Subject' to get a list of available fields that can be shown.

(Note that the editor's status bar also shows (and lets you change) the account from which you're writing, so in that sense the From field may not be required)