Welcome to our community! I'd be happy to help you get this issue resolved but first need to get some specifics. When you attempt the update do you receive an error or failed message? Is it any app in the store or a few specific apps/updates?
Error 500 is a server error. If you're consistently getting that, it's possible this is a really bad DNS entry problem that's propagated throughout your local network. Recommend:
1. Powering off all devices connected to the modem via wifi or ethernet cable.
2. Powering off the modem by unplugging the power cord from the wall.
3. Wait 60 seconds, then plug the modem's power cord back into the wall.
4. Wait another 60 seconds or so and repower your devices.