This is a bug and will be fixed in the next patch.
A workaround until then is to hover over the Back Translation entry and click on the edit icon (looks like a pencil). This will cause the details of the setup to be displayed and cause the error message to close.
The problem is that Paratext isn't handling the setup for a back translation that doesn't have a model text correctly until the details are displayed.
Sorry for the inconvenience. We are in the midst of creating the next patch, but because of testing that will be required before release, it will be at least mid-July before the patch is ready.
John Wickberg
Paratext Support