I am getting a "server not found" error when trying to sign up using my webform I have created. On step 3 I have left the success url blank and used the automatic version code.

1. Have you checked your My Account > Account Details > Company website field? I expect it is showing as "Empty" which is causing that error. So simply insert that website address into the Company website field and that should sort your webform.

If you have used the automatic version code any changes you make subsequently should auto-change on your webform.