There might be a few ways to resolve this issue when an app is stuck on Download pending... try each solution in order to see if it resolves the issue.

1. Force Stop Google Play Store app

1. Open Settings  
2. Select Apps.
3. Select Google Play Store 
4. Select Force Stop
5. Re-open the Play Store app

2. Clear Play Store download queue

1. Open the Play Store app
2. Swipe-in from the right edge of the screen, and select My Apps & Games
3. To cancel an app from downloading, select it and touch the X (cross icon) near the download progress bar to cancel the download.
4. You can do this for all apps to clear the download queue.

3. Clear Play Store app data

1. Open Settings  
2. Select Apps.
3. Select the Google Play Store. If you are unable to find the Google Play Store from the list, select the menu button (three dots) and search Show system apps
4. Once you are in the Google Play Store app info page, select Clear Cache and Clear Data. If you are on Marshmallow or higher Android versions, select Storage then clear cache and data.

Reset Apps

If none of the above solutions works out for you, try resetting the apps. This will reset all your apps preferences. Make sure you backup settings of all your important apps before resetting.

1. Open the Settings app.
2. Scroll down and tap on Apps (or Applications) menu.
3. Press the 3-dot menu button (in the top right probably).
4. Tap on Reset app preferences.
5. Confirm on the next screen by tapping on the Reset button (could be Reset apps).