When I turn on my iMac, it says it cannot connect to my wireless keyboard or mouse. I don't have either and it is connected to my USB keyboard and mouse

Did you ever have or was there at sometime wireless appliances connected to computer? If yes, then more than likely they were connected by Bluetooth. Go to Bluetooth preferences and see if anything is listed in Devices. If any are then click on X on the right as you hover over the device name then remove on the prompt that ensues.