Why not "chleb"?
Chléb is accepted. It is usually necessary to tell us your complete answer to be able to tell what is wrong.
What's the difference between "chleb" and "chleba". Maybe the size or ....?
Not the size. Chléb is more formal, it is what will likely be written in the bakery or grocery store where they sell it.
Chleba is what people normally use when talking colloquially.
why "kde je to chléb" is wrong?