If you were redirected to the Playstation Store after accepting a Standard Pack Friend Invitation, there is an effective workaround that will grant you access.
1. Open the Playstation store
2. Scroll down to "Games"
3. Search for "Fortnite"
4. In the list of Fortnite Editions shown at this point, the Standard Pack should be displayed as "Free".
5. Select this Free Standard Pack and download the game.
Once you have selected and downloaded this pack, you're all set. We apologize for any confusion.