Why is it true to say that calcium carbonate
has both ionic and covalent bonds? the

Calcium carbonate (CaCO3), essentially, is an ionic compound having the bivalent calcium and carbonate ions.

But the carbonate anion is a polyatomic species. The carbon atom is bonded to all the three oxygen atoms by covalent bonds - two carbon-oxygen single bonds and one carbon-oxygen double bond. Owing to resonance, the three carbon-oxygen bonds are also of equal length in the carbonate anion.

Any salt having a polyatomic ion invariably contains both ionic and covalent bonds.