Sometimes when you are trying to install or update some apps from Play Store, occurred some error like :error 921 ''can't download or update app", Let's pass it ...
All you need to do is refresh the Google account connection in the Play Store. This means you simply have to go to the
2-Apps or App Manager
3- find Google Play Store
4- clear the cache.
Try to download the app again after this and if it still doesn't work, go back and also clear data. It's not a bad idea to
5-clear the cache of the Downloads Manager app .
Then try downloading again. If it still doesn't work, repeat the previous steps and then go to the
6-Accounts and Sync settings on your device,
7-remove your Google account
8-then re-add it.
Rebooting your device at this point can also be beneficial before accessing the Play Store again.
but if you are getting another error and these steps don't solve the problem, check common Play Store errors and their solutions .