Border Layer cannot be created

Thanks for sending a sample file.  In this case FAB 3000 doesn't detect 0-width lines (typically border outlines in Gerber consist of traces with width).

Please perform either of the following:

1. Create a Dcode (any empty dcocde), and assign a width.
2. Go to Edit / Properties..., select border objects, and assign a width.
3. Now Border Detection will work.  

Here's a quick tutorial showing how to perform that task.

https://numericalsoftware-update.s3.amazonaws.com/tutorials/2016-08-24_11-43-56-line-border-automatic.mp4

Note:  We will search into why 0-width objects are not used in Border detection, and if possible create an update that accepts 0-width objects.

Lastly, here's the most bullet-proof way to create a border:

1. Create a new layer and assign the Layer Type as Border.
2. Copy all border objects to it.
3. Now generate Border (menu: Fabrication / Detect Border Automatic) and you will be fine.

Here's the movie tutorial showing those steps (with a bonus PCB realistic preview):

https://numericalsoftware-update.s3.amazonaws.com/tutorials/2016-08-24_11-49-00-create-border-pcbpreview.mp4

Let me know if that helps.

Best regards,

Simon