So if your having problems with your samsung galaxy s5 not downloading apps onto your phone theirs 2 main reason why this is happening. The first one is that you need to go into the system settings and see how much storage space you have left. If you have used up all the storage on your galaxy s5 you wont be able to download anything else onto it so check that first.
The second reason this is happening is because your not connected to wifi. Alot of the time you will need to be connected to WiFi in order to download an app.