dell optiplex 7040 sff amber

What have you actually tried for troubleshooting?  A few ideas in the order they're presented.

First try, replacing the size 2032 CMOS battery and do a BIOS reset.

Dell BIST Test

Blinking amber can mean bad PSU if still no luck.  At least it's not solid amber, which indicates a motherboard problem.

@bradthetechnut​ The BIST LED lights up green and the fan spins. Changed the battery but still the same, Getting into bios doesn't work. Can't get anything on the screen. Not even the logo. Lights up white once, then starts flashing.

A bad PSU on its way out can still spin the fans when the test button is pushed.  Some have even tried pushing the power button at the same time pressing the PSU test button.

You don't need to get into BIOS to do a BIOS reset.  It's per the instructions I linked.  Preferably the jumper method if your MB has it.

The above are steps to try before ruling that it's likely the PSU.

BIST: if LED must be solid and not flickering or flashing. The power supply fan turns on.  then the power supply is delivering power to the computer.

7040 SFF mobo does have a clear cmos jumper pins

it is at the bottom of mobo below the blue PCIex16 slot. labelled as CMCLR

move the blue jumper to those pins x 5 sec then move back

https://i.ebayimg.com/images/g/3pYAAOSwdSBin7qr/s-l1600.webp

your video is too short

check the amber blink pattern

Amber LED blinking pattern : The pattern is 2 or 3 blinks followed by a short pause then X number of blinks up to 7. The repeated pattern has a long pause inserted in the middle. Example 2,3 = 2 amber blinks, short pause, 3 amber blinks followed by long pause then repeats.

https://www.dell.com/support/manuals/en-us/optiplex-7040-sff/opti7040_sff_om/diagnostic-and-power-led-codes?guid=guid-7d615d96-eb00-4be6-b8dc-191949e0f418&lang=en-us

Lol, 4x4 does not = 8,  and dual channel would work better unless someone already damaged the slots.  Use an air duster and blow out the DIMM slots to see if it may help.

From what I gathered from the video, the blinking is simply steady.  I viewed it again.  No blink pattern.  I'm sure the video would've been longer if there was a blink pattern.