TKPC621, please be sure the SSD you are going to use as cache does not have partitions on it
If there is any partition on the SSD, then Intel(R) Rapid Storage Technology will not detect it as an option for caching hence it will not show the option for Intel® Smart Response technology acceleration.
Which version of RST are you using?
There are newer RST versions however I would like to find out which one is the best for your system. So we would need to start updating the software.
Please try installing version 11.7 https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=22194 Intel® Download Center? Let me have the results.