When USPS tracking says "Unfortunately we are unable to retrieve your tracking results at this time" it just means something has gone wrong on the USPS tracking web servers.
The only thing you can do to fix the issue of "Unfortunately we are unable to retrieve your tracking results at this time" is to wait and try again in a few minutes or so.
Sometimes the servers get overloaded because of people tracking the packages through USPS tracking and then it gives that error.
Whenever I get that error I just wait a few minutes and then try tracking the package again and it usually works.
Sometimes it may take as long as 10 to 20 minutes of waiting but eventually the tracking should work again.
When USPS tracking updates to say your package has been accepted then the package will be on it's way to you and when the tracking says out for delivery today then you'll get the package that day.
USPS tracking is not very reliable though and not in real time so checking more than once per day will not show any updates on your package.