Remove the un-used SSD half of the slot and then try to create the Virtual DIsk Group with SAS drives as you want in Pool. It's possible that Controller is thinking SSD drive available in system without Performance Tier License so it's not allowing to create new VDG. So after you create the new VDG in Pool A then insert the SSD and try to configure SSD read-cache
Please refer the below forum discussion as well where I have fixed similar issue,
Hope this helps!
Regards
Subhajit
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