If you have quad rank DIMM you can only populate 2 DIMM per channel. Please refer below link for details. Can you try without populating DIMM slot with green release levers and see whether it is working without any issues? [URL] [URL] 
Hello i have a dell poweredge r720 and i am looking to upgrade the ram. i have 24x8gb 4rx8 pc3l sticks and when fully installed showed an error of course. i was wondering what config i would need to be able to use them, it currently has 4x16 sticks but was unsure if i was able to fully popluate it with 8gb sticks.
Ram config r720