if the symbol for cobalt Co wewre written as CO what would be wrong with it

Because in a symbol, only first letter should be capital. So, Co stands for element cobalt but CO stands for carbon monoxide. If we write CO instead of Co for cobalt, then that would be wrong.

Because if we writeCO it will become carbon monoxide

The symbol for Cobalt cannot be written as CO because CO is the symbol for a poisonous gas called carbon monoxide.
If cobalt was written as CO, it would have been misunderstood as carbon monoxide instead of Cobalt.

The symbol foe cobalt is Co whereas, the symbol CO represents Carbon Oxide.