Device or computer?
Device:
Do you have access to another network where you can try?
Try a restart.
Try a forced restart.
Computer:
Quit the App store, reopen and try again.
Also try the Safe Mode.
Safe Mode - About El Capitan
also Sierra
Safe Mode Startup – El Capitan. also Sierra
If necessary, contact iTunes Support. Scroll to the bottom of the page in the 2nd link.