Solution:
More of the information about SSPI programming can be found at MachXO2 Programming and Configuration Usage Guide (TN1204). SSPI port is active when the device is in blank or erase state. However, this port is defaulted as not active or disable at Diamond Software set SLAVE_SPI_PORT to disable (default value). You may use Spreadsheet view to ENABLE the SLAVE_SPI_PORT to keep your SSPI port active in user mode. In this case, you can start communicating with SSPI when SN is asserted.