Hi MSiLight,
Thanks for contributing to the Apple Support Communities.
You mention that you can't sign in to the App Store or iTunes Store on your Mac after updating to El Capitan. It looks like you've already tried a lot of troubleshooting found in other threads. Knowing the results of what you tried can help with making some additional suggestions.
For example, there are some things I'm wondering about the issue that can help guide our troubleshooting:
- Can you sign in to the App Store/iTunes Store with your Apple ID on another Mac?
- Can you sign in with a different Apple ID on your MacBook Pro?
- Can you sign in with your Apple ID on your MacBook Pro from another network (like a library)?
If you haven't already tested to find the answers to these questions, give them a shot!
Additionally, be sure to try testing in another user account on your Mac--How to test an issue in another user account on your Mac
Let us know your results and what else you've tried and we may be able to suggest some additional steps.
Best Regards.