Mixing RAM most of the time can be troublesome.  For best results only use the same brand, size and spec's.  

This is not just a "Dell" issue.  I have an ASUS ROG (Gamer) motherboard and it will not work with two different brands of memory of the same size and specs.  (I have two Corsair Dominator 8 GB memory modules and two GSkill of the same size and speed and it will boot with either installed but not both).

Per the online specifications page, 48GB is not tested/validated by Dell. If you were going to try it, you should be using our OEM memory sticks.

Closest to the CPU =
Slot 4 black XMM1 (YXC0V 16GB,DDR4,2400MHz,2RX8)
Slot 5 white XMM2 (M0VW4 8GB,DDR4,2400MHz,1RX8)
Slot 6 black XMM3 (YXC0V 16GB,DDR4,2400MHz,2RX8)
Slot 7 white XMM4 (M0VW4 8GB,DDR4,2400MHz,1RX8)

M0VW4 8GB,DDR4,2400MHz,1RX8 =
Hynix HMA81GU6AFR8N-UH, HMA81GU6CJR8N-UH
Kingston KM0VW4-MIB, KM0VW4-MID, KM0VW4-MIE, KM0VW4-MIH
Micron MTA8ATF1G64AZ-2G3B1, MTA8ATF1G64AZ-2G3E1
Samsung M378A1K43BB2-CRC, M378A1K43CB2-CRC

YXC0V 16GB,DDR4,2400MHz,2RX8 =
Hynix HMA82GU6AFR8N-UH, HMA82GU6AJR8N-UH
Kingston KYXC0V-MIB, KYXC0V-MID, KYXC0V-MIE, KYXC0V-MIH
Micron MTA16ATF2G64AZ-2G3B1, MTA16ATF2G64AZ-2G3E1, MTA16ATF2G64AZ-2G3H1
Samsung M378A2K43BB1-CRC, M378A2K43BB1-CB1

There are only 4 Ram Slots.

I would advise buying 4 x 8 or 4 x 16 matched sets.

Aurora R6 Ram Slots

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_alienware_dsk/alienware-aurora-r6-desktop_service%20manual_en-us.pdf

https://www.amazon.com/Hynix-PC4-19200-2400MHz-288-Pin-HMA81GU6AFR8N-UH/dp/B075G3RD21

https://www.amazon.com/Hynix-DDR4-2400-Non-ECC-Memory-HMA82GU6AFR8N-UH/dp/B076849PN2