Hi Warren and welcome to free pc help.
If you take the side panel off your case at the bottom of the motherboard there should be a battery (round one found in watches and clocks) if you pop the spring bit next to it, it should fall out - try and reinsert it and boot the system.
see if you get the same error - you do not have to have a battery in but it will not save any settings on the BIOS without one inplace.
have you taken the battery out recently or moved any of the jumpers?