It is very confusing only by the technical name CO2 and CO already
Carbon dioxide in short CO2, carbon dioxide is non-toxic and you find it in carbonated drinks and in the air we breathe. However, because in gas form its heavier than air and it can expel oxygen from a space. Compare the carbon dioxide with water, water is non-toxic as well, underwater you still can't breathe.
Carbon monoxide is a poisonous gas abbreviation CO, which is produced in malfunctioning heaters. Infamous are ancient geysers and facade heaters that are not connected to a smoke outlet or on a poorly functioning chimney.
Unnecessary to say that you cannot use a much cheaper Carbon monoxide gas warning system for Carbon dioxide.