in which the port name is actually "CLK_66M_n" in the RTL code.
Constraints in scoped XDC (IP XDC, OOC XDC, etc.) should use the object name and hierarchy within the specific scope. These warnings can occur when the hierarchy of the object name does not match the scope.
For example, for a register named a_reg in an IP instance "a_inst" under the top level, the following constraint in the IP XDC causes the above Critical Warning.