To get 8188 into Monitor mode while using RTL8188, use the following setup instructions:

iwconfig ra0 mode monitor

But the print appears as follows:

Error for wireless request "Set Mode" (8B06) :
    SET failed on device ra0 ; Operation not supported.
device ra0 entered promiscuous mode

Note that this operation is not supported because iwconfig does not support it, but check that iwconfig supports setting modes

~ # iwconfig --help
Usage: iwconfig [interface]
                interface essid {NNN|any|on|off}
                interface mode {managed|ad-hoc|master|...}
                interface freq N.NNN[k|M|G]
                interface channel N
                interface bit {N[k|M|G]|auto|fixed}
                interface rate {N[k|M|G]|auto|fixed}
                interface enc {NNNN-NNNN|off}
                interface key {NNNN-NNNN|off}
                interface power {period N|timeout N|saving N|off}
                interface nickname NNN
                interface nwid {NN|on|off}
                interface ap {N|off|auto}
                interface txpower {NmW|NdBm|off|auto}
                interface sens N
                interface retry {limit N|lifetime N}
                interface rts {N|auto|fixed|off}
                interface frag {N|auto|fixed|off}
                interface modulation {11g|11a|CCK|OFDMg|...}
                interface commit 
       Check man pages for more details.
~ # 

Then I looked at the driver source code and found that the driver source code turned off Monitor mode by default, as follows:

CONFIG_WIFI_MONITOR = n

Rewrite N to Y, then recompile the driver, burn it to the device, and the test can be set to Monitor mode as normal.