sql server management studio enable encryption
SSMS GUI expects you to Create your server certificate before you mange database encryption. 

Ref: CREATE CERTIFICATE (Transact-SQL)

Once you create the server certificate you you will be able to chose from the drop down box. 

CREATE CERTIFICATE testencryptioncert   
   ENCRYPTION BY PASSWORD = 'pGFD4bb925DGvbd2439587y'  
   WITH SUBJECT = 'Testing Encryption',   
   EXPIRY_DATE = '20201031';  
GO


Now the above certificate will show up in the drop down list of the GUI.