Unable to download application : App store error's solution

Unable to download application : App store error is usually a transit error for the communication between the device and the Apple store.

The solution : Try till you succeed 

1. The error shows two options done and retry. 
2. Choose done.
3. Then click on the app once again.
4. If it again points to the error. Follow step 2 
5. Repeat these steps until the app shows waiting or loading.