Google Play Store downloads won't start I ran a speed test and I'm getting a full 4Mb/s. However, when trying to download an app, it's stuck on the indeterminate progress"Downloading" bar. 20 mins later it gives me an error saying it couldn't be installed. It's not just one app, it's EVERYTHING I try to download.
You could try the following steps to troubleshoot the issue:


Ensure components used by Playstore are not disabled


Check if download manager, Google play services or Google services framework is not disabled or frozen by package disabler apps.
Freezing/ or disabling such can result in failed downloads.
Re-enable them as necessary






Removing and re-adding Google account


Sometimes perfoming this this step acts as a way of "refreshing" Google Apps including Playstore and may fix failed downloads.






Uninstalling Playstore updates.


You can try removing Playstore updates and then update to latest version which can solve the issue, as a certain version can have bugs. Do this:




  Reinstall Play Store updates
  
  
  Open your device's main Settings app.
  Touch Apps or Application manager (depending on your device, this may be different).
  Touch Google Play Store > Uninstall updates .
  When prompted to change the Play Store app back to the factory version, touch OK.
  Open the Google Play Store app Google Play .
  Go back to your device's home screen. As long as you have a strong Internet connection (Wi-Fi or mobile data), your Play Store app will
  update to the latest version within a few minutes.
  





Clearing Playstore data


You can restore Google Playstore to default by clearing its data, which can hopefully solve the issue of failed downloadsn




  Clear cache and data
  
  
  Open your device's main Settings app.
  Touch Apps or Application manager (depending on your device, this may be different).
  Touch Google Play Store > Clear cache > Clear data .
  




Source: Google Playstore Help - Play Store won't load or download apps