I just ran into this problem this evening with only 2 of the 11 book line items I was submitting. I found if I manually entered 12/18 (December, 2018) for the dates that were actually early next year (1/19 or 2/19), it would let me select the Modern tier *and* price it with the other 9 line items without "call for price" showing up. Of course, I had to manually enter those dates and all the data in the fields after that, but I knew exactly what to put for the variations since I had blindly went through the selections before finding out the tier wasn't selectable. So, if you get this problem, try backing up and playing around with the date.
