Code signing and avoiding error 65

I have one  existing project which works correctly in xCode and Appcode.   However  if I create a new project in either XCode and Appcode I get the dreaded 'Build finished with Error:65.'   plus some detail  telling me  have a code sign error.

If I go into xCode I  can either tell it that I am an 'iPhoneDeveloper' or 'Don't Code Sign'  for my new project.

• If I choose  iPhoneDeveloper  AppCode tells me  that the identity doesn't match any valid, non-expired certificate/private key pair in my keychain ( which is true,  I don't need to be registered for the work I do)
• If I  choose 'Don't Code Sign'  AppCode tells me  code signing is required for product type 'application'

In xCode it  works anyway -  it's only AppCode  that does not work.

However 'Don't Code Sign' is  what I have set in my working project -  and I can't see any difference whatsoever.  All I know isthat my existing project works, and my new project does not work.

So  was I just lucky first time round? - what  can I do to make a new project work?     My new project can be  trivial -  it doesn't need to have anything in it. I  would expect this issue to hit  any newbie  who is trying out xCode/AppCode  for the first time so is worth solving.