<error message>
Error: Can't place pin to location
Error: Can't place VREF pin (VREFGROUP_**_**) for pin of type bi-directional with SSTL-18 Class II/O standard at location
Error: Too many output and bidirectional pins per VCCIO and ground pair in I/O bank when the VREF pin (VREFGROUP_**_**) is used on device
-- no more than 9 output/bidirectional pins within 12 consecutive pads are allowed when the voltage reference pins are driving in, but there are potentially 10 pins driving out

This error message is output because if there is a voltage reference pin that is input to the VREF group of the device, it is not possible to assign 9 or more output pins and input/output pins to one VREF group.

You need to set OUTPUT_ENABLE_GROUP for the corresponding pins.
This setting is considered to be the same output enable group and is a setting that does not violate the maximum number of output pins in the VREF group.

The specific settings are as follows.
set_instance_assignment -name OUTPUT_ENABLE_GROUP -to

can be any number, but must be set to the same number to be considered as the same output enable group.