Sometimes your package may have arrived at your post office and was or is available for pickup but they may not have gotten around to putting a yellow slip in your mailbox or a key to a parcel locker in your mailbox.
That is true if it's a busy post office and people do make mistakes.
You should go into the post office with the tracking number and ask them to help locate the package as sometimes that has happened to me and it's been there but they just forgot to deliver it to me or forgot to put a slip in my mailbox.
Or if you get mail delivered to your house they might have missed your package and forgot to put it on the delivery truck.
When USPS tracking info says Available for Pickup it means your package has arrived at your local post office where you get your mail.
If you have a post office box then they will usually put a yellow slip in your mail box to bring into the post office at the counter and pick up your package.
Or they might put a key in your mailbox for one of those parcel lockers in the lobby of the post office.
Being available for pickup doesn't always mean that it's ready for pickup though because sometimes It just means it's at the post office.
But if it says available for pickup you can usually go to the post office and get the package as it should be there.
When it has been delivered the tracking information should update to say delivered.