Hi,
The error cannot be reproduced. I enable the following assignments and the software reported that this node does not exist in the design.
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst0|dd_green"
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst0|dd_red"
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst0|dd_blue"
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst1|dd_blue"
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst1|dd_green"
set_instance_assignment -name FAST_INPUT_REGISTER ON -to "TMDS_Receiver:TMDSReceiver_inst1|dd_red"

Could you help to check?
Thanks.