Apple Mail doesn't display the signature dropdown in new email from line to select manual Signature
Go to the Library folder of your user folder. It is hidden, so press command+shift+. to unhide all folders. 
Go to 



  Mail/V7/MailData/Signatures.



Move the two .plists to your desktop 


Restart your computer, open the application, and test. If it works okay, delete the plists from the desktop.