You may have an old firmware. Please go to our FTP location and download the IMR firmware. You will need a Dos bootable USB stick to save the files on. Once you save the files, boot to Dos prompt.
https://www.supermicro.com/wdl/driver/SAS/LSI/2008/iMR/
Run the chkadd.bat file to capture the SAS address. Read the adpinfo.txt file for the address and write it down. Boot to Dos prompt again on the server, then run up2imr.bat to update. Reboot, then run setsas.bat and enter the SAS address. Reboot once more and try to create your RAID.