Ammonia forms the complex  ${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}$  with copper ions in alkaline solution but not in acidic solution. The reason for this is:

In alkaline solution $Cu{\left(OH\right)}_2$  is precipitated which is soluble in excess of alkali   

Copper hydroxide is amphoteric substance

In acidic solution hydration protects $C{u}^{2+}$  ions    

In acidic solution protons are coordinated with ammonia molecules forming $N{H_4}^{+}$  ions