Solution:
Internal Vref Generator is not supported on MachXO3. MachXO3 SysI/O Usage Guide FPGA-TN-02056 was initially referenced with MachXO2 SysI/O Guide FPGA-TN-02158 which supported Vref generator for SSTL and HSTL buffer types. However, this is not supported on MachXO3.