In every case of this I have seen it has been because the field is already on the layout. It may be tiny or hidden by another object.
In the layout designer hit the F4 key and you should see the properties window appear on the right. At the top is a dropdown where you can select all objects on the layout. Look for the field in question in the list and select it. Once selected it will highlight the field by placing white squares around it and let you find it on your layout.
When I am in the design layout, the field is there. However, when I go to Tools Define Fields to edit the field it is not there anywhere.
Other than a system field, as Roy pointed out, another option is that the Label on the layout might have been changed. In Design Layouts, make sure you're looking at the name in the field box, not the label text next to it.