I note the OP specified this was his "desktop PC", not a laptop.
So assuming it really is a desktop, and the error is real, it can mean 1 of 2 things. Either your CMOS battery is failing, or you have this PC connected to a UPS and the UPS is reporting those batteries are too low.
The CMOS battery is typically a CR2032
"wafer" battery that is mounted on the motherboard. These typically cost a couple dollars and can be found at most battery counters. Unplug the computer from the wall, open the side panel and touch bare metal of the case interior to discharge any static in your body. Then carefully pry out the old battery, noting the polarity.
Do not touch the new battery with your bare fingers. Skin oils promote corrosion and attract dust. I put a clean sock over my hand. Insert the new battery, noting again the proper polarity.
Make sure the interior of the computer is clean of heat trapping dust while in there.
Connect power and boot directly into the BIOS Setup Menu. Set date and time and make sure your boot order correctly points to your boot drive. Then Save and Exit to boot normally and see if you get the error.
If you have your computer connected to a UPS (and kudos to you if you do), then note UPS batteries typically need to be replaced every 3 - 4 years. You will need to check your UPS manual for battery specs and instructions to replace. Note you do not have to buy UPS replacement batteries from the UPS maker. You can, but they will cost you a premium.