Cannot install any Custom ROMs on Xperia Ray ST18i: assert failed: getprop Status 7
You have to install the boot image that is included in the Cyanogenmod ZIP file an flash that in fastboot:

Connect your device with Vol-Up While plugging in the USB-Cable

fastboot devices


Seeing your serial number means we know all is good. 

Now type these commands:

fastboot erase system -w
fastboot erase boot


then copy the boot.img in the same folder as fastboot.exe and type 

fastboot flash boot boot.img
fastboot reboot


Now press the Vol-Down Button to enter the Recovery Mode again and install your image