While downloading an app in Google Playstore, you might get
Can't install app
** can't be installed. Try again, and if the problem continues, get help troubleshooting. (Error code: -504)
If you do then the culprit is likely a slow internet connection. Try enabling then disabling Airplane mode or changing your source of internet connection.
Other fixes to consider
* Go to Settings > Apps > All > Google Play Store > Clear cache and Clear Data
* Again, from All > Google Service Framework > Clear cache and data
* Ensure you have sufficient storage space on your Phone storage