I am getting an error message "No trades are currently allowed"

Enable Thinkorsim for your account. See step 2 of the instructions Setup Instructions

When I attempt to authorize my account I get an error page

This is normal. Copy the result from the address bar and paste into the Excel window.

I need to re-authorize for a different account

Delete the following values in column B from the Settings tab of your spreadsheet: Access Token, Refresh Token, Id Token, Access Token Date/Time, Refresh Token Date. Now go to the Stocks tab and click Get Quotes. You should be asked to re-authorize and you may select another account.

Can you provide a list of all the fields that are available via the API?

You can research the Schwab API here: https://developer.schwab.com/