Apple - "Unable to obtain authorization for this operation" when trying to reset my default keychain

I was able to fix this issue by restarting my computer in recovery mode and using disk utility's First Aid to repair all volumes and the disk itself. I don't know which repair actually fixed the issue, but when I restarted I was able to view and save items in my keychain.

You can enter recovery mode by restarting your computer and holding down the keys command and r. Once you're in recovery mode you can select "Disk Utility" to get to the First Aid tool.

Once you're here you can click on each of the volumes (the indented names on the left. E.g. Macintosh HD) and then click the First Aid button.

I repaired both of my volumes and the disk itself (the APPLE SSD SM...). The first aids for the volumes and the disk itself do different things.

After you're done, restart your computer and try to use/view your keychain. Cross your fingers and hope you don't have to do more googling!

Running Disk Util also fixed this exact same problem for me. I have 10.12.6 that was working fine, until I came in to work one morning and suddenly nothing worked - Box and several other services kept demanding authentication and failing, claiming they could not access keychain. Trying to reset keychain resulted in same problems shown above in this post. Surprisingly, Disk Utility fixed it all.