Question

If the symbol for cobalt (Co) is written as CO what would be wrong with it

Answer 1

The symbol for Cobalt is Co and not CO because CO can be misunderstood as carbon monoxide(CO).

Also, by convention, when the symbol of an element contains two letters, the first one is capitalized and the second is not. Other examples include Be, He, Fe, Pd, Au, ..

Answer 2

The wrong thing in writing cobalt as CO is that we can't write o as O because O represent Oxygen and symbols representing an element with two letters have first one as capital and next one should be in small letter.
Therefore we should always write cobalt as..Co not CO!