It would help to have the details of the computer (system, motherboard, CPU) and details for the memory modules (brand and model/part number).
Several things could cause this issue, though.
Wrong type of memory modules being used (partly incompatible with system).
CPU not mounted correctly, or pins bent or missing, or dirt/debris between CPU pads and pins.
BIOS not up to date to recognize newer memory module types.
Thanks for the extra details. Although the memory part number (starts with "H" or "K") would be more useful.
The msconfig should have the memory box UNCHECKED, as shown in the screenshot, as this setting will not limit memory available to Windows. So the current setting is OK.
One thing to check, is to make sure the latest BIOS has been installed. The latest BIOS is available from here. Select the BIOS tab from there and download and install the latest BIOS if necessary.
Remove the CPU and thoroughly check that it and it's corresponding socket are free from any dust, dirt or any other kind of debris. Also check the pins and pads for signs of damage, bent pins etc. If all OK, make sure the CPU is mounted carefully and evenly. Uneven mounting can cause loss of contact between some pins and pads which in turn causes numerous issues such as loss of available RAM.
It is assumed that you have 2 x 8 GB memory modules from the same kit?
If so, then check here to see if your modules are listed. If the modules are listed then the memory is compatible with your system. If the modules are NOT listed, then the modules might not be (fully) compatible. However, your modules might still be compatible despite not being listed as not every possible combination of modules and motherboards is tested and listed.
If the two memory modules are NOT from the same kit, then the issue might be due to inter-module incompatibility. In this case, try the modules in different memory slots. Increase DRAM Voltage in BIOS a bit, but don't exceed 1.35 V. You might (also) have to ease off DRAM Timings a bit in BIOS. No guarantee that the modules will work together though (if they are not from the same kit) despite trying all these options.
brispuss mentioned it in their post, but try changing the slots the RAM are installed in. They should be in alternating slots, not right next to each other. They should probably be in the second and fourth RAM slots, counting from the CPU side to the front of the case (left to right). This is a basic thing, that shouldn't cause a big issue if wrong, but it could be a problem.
Also, you can try with just one stick of RAM, and then change it out for the other. If the system doesn't work with one stick , but works with the other, than one of your RAM sticks could be bad. Try this using the same RAM slot. Things listed above are also worth a shot.