Are you using NON-ECC RAM? You can try pulling the CMOS battery out for 15 minutes then reinstalling the battery. Are you using Crucial RAM as Dell systems are very picky about RAM makes.

One interesting point though. The specs listed on the support page are different from the spec sheet here. The support page lists 2 x 32 GB as compatible but the spec sheet here only shows 4 x 16 GB as compatible for 64 GB total.

The unit the I have have an 8gb ddr4 stick from Samsung.  So I bought some 32gb DDR4 and I thought they're compatible.

*I bought x2 Ddr4 Samsung RAM.

@B.Hxllywxxd 

32gb x 2 isnt supported

Crucial PC4 21300 2666MHz B0892PZH6Y

Optiplex XE3 Owner's Manual states max supported RAM per DIMM is 16GB.

Ok I forgot to post the link but on this support page it lists 2 x 32 GB as compatible. That must be a typo as both @bradthetechnut and @speedstep state that it is not a compatible configuration.

I think it's definitley a couple of typos as the configuration isn't working.  It's not a mix up with the mini tower either.  Owner's manual for mini tower also states 16GB max per DIMM, as does spec sheet.

Official Dell/Crucial part is very expensive.

 Dell 16GB UDIMM 2666MHz AA101753 B07M98ZQVM

https://www.dell.com/en-us/work/shop/desktops-all-in-one-pcs/optiplex-xe3-small-form-factor/spd/optiplex-xe3/cto01oxe3sffus?view=configurations

@speedstep @bradthetechnut @JOcean 

Thanks for spending your time assisting me with my problem.  After around 1 week of research, I figured out that the issue lies with the Processor itself.  Since the i7 8th gen is limited to 64GB (total) of RAM @ 2666mhz. Currently I'm running (x2) 16GB @ 2666 plus (x1) 8GB @ 2666. 

Since my whole house is running Macs/Chrome Os, I was configuring this unit to be my whole home server running Truenas Scale with Plex/NextCloud/HomeAssistan running in Dockers

Currently, it has an (x2) 10G Nic, (x3) 8TB SSD and about to grow more once I can figure out how to install a driver for my (x8) pcie msata multiplier in TrueNas.  But this is the OS's problem now not the hardware.

You're welcome.  : )

May be with any luck @speedstep or somebody will have an answer for your driver quest (msata multiplier in TrueNas).

@bradthetechnut 

@B.Hxllywxxd 

The writers of True NAS say DO NOT USE port multipliers.  Definitely NOT supporting TRUE NAS or any other 3rd party software let alone hardware.

truenas multiply your problems with sata port multipliers and cheap sata controllers

In the last year or two, we've had a resurgence of users asking about SATA Port Multipliers and cheap SATA controllers.

Please, do NOT use port multipliers, and use cheap SATA controllers only after extensive research.

SATA controllers and SATA Port Multipliers are some of the cheapest hardware, designed to make a number of hard drives accessible to Windows. A port multiplier takes a single port of a SATA controller and multiplexes it between several drives.

Windows typically only accesses one or two drives at a time, which means that the needs of Windows users are often relatively limited. However, ZFS often accesses data on all drives simultaneously, such as during scrubs and resilvers. This creates a lot of stress on I/O controllers, and tends to expose any problems.

Cheap SATA controllers are often built as PCIe x1 cards, to make them compatible with the largest number of mainboards. On PCIe 2.0, a single PCIe lane is capable of 500MBytes/sec, or at best, two HDD's worth of transfer speed. Yet, many cheap SATA controllers (Syba SI-PEX40064 example) have four SATA ports on them. There is insufficient capacity to talk to all four drives simultaneously.

Worse, often, this is combined with SATA Port Multiplier to offer cards handling as many as 10 drives, so clearly there isn't enough bandwidth there.

So far, all of this assumes that all the hardware works swimmingly well, and even so, it already is incompatible and bandwidth limited.

Unfortunately, many cheap SATA controllers are a bit dodgy, and do NOT work well. Because they are price-driven by the PC market to produce the cheapest product, many of these cards use dodgy controllers which may be knockoffs of already poorly-designed controllers by companies who have put out something that is just good enough to work acceptably under Windows. Often, these chipsets do not work particularly well under FreeBSD and Linux, especially when stressed by the massive I/O loads imposed by ZFS.

Worse, SATA Port Multipliers require both a SATA controller that works perfectly, AND a host driver that understands how to control the Port Multiplier correctly. The number of working, trustable combinations of these that exist are relatively few. The fine folks over at BackBlaze and 45Drives built their first generations of storage appliances with these after exhaustive testing that only found a few good combinations. They have posted significant discussion of their experiences over at http://www.45drives.com/blog/uncategorized/our-biggest-design-decision-direct-wired-or-backplane/
which is worth reading before you choose to go down the Port Multiplier path.

Almost all of these "solutions" have been more expensive to buy new than it costs for a used LSI HBA on eBay, where the PERC H200's and PERC H310's are often available for $30, and provide eight fully supported lanes and can use SAS expanders for additional drives beyond that.