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.