As the error following the slot there could be some issue on the slot. Can you try connecting DIMM to A1,A3,A4,B1,B3 and B4 and see whether all memory modules are getting detected. 
We are users of the Dell Poweredge R730 server. We currently have one CPU (Intel(R) Xeon(R) CPU E5-2640 v4 @ 2.40GHz,  2400 MHz) and 64.00 GB of RAM (4x DDR-4, 16GB,  2133 MHz modules). Just bought another CPU and two more identical RAM modules (2x 16GB 2133MHz Modules). Is a configuration with a total of six identical RAM modules and two identical CPUs possible?  Which RAM slots should we choose ?  Thanks in advance.
DELL POWEREDGE R730 CPU and MEMORY UPGRADE