You cannot assign SSTL, HSTL and HSUL outputs to VREF pins in Arria® V devices.
The Quartus® II software version 11.1SP1 incorrectly allows you to assign these I/O standards to VREF pins when there are no SSTL, HSTL and HSUL input assignments in the I/O bank.
VREF pins in Arria V devices are dual-purpose, so they can be used as regular I/O pins when not used as a voltage referenced input power pin. However, when using for regular I/O functions, they do not support SSTL, HSTL, or HSUL standards.
The Quartus II software version 11.1SP2 will generate the above error message if you try and assign SSTL, HSTL and HSUL outputs to VREF pins in Arria V devices.