use any of the things that can be used from your backpack and you will able to claim your reward from mail. since you re having fragments to be claimed, you might need to use the fragment refinement from ninja menu.

I tried

Hello,
Kits's second recommendation is correct. Please try using the Fragment Refining interface in the ninja section to clear out some old five star units. There is a finite space for these additional fragments. If you clear out enough old, unused fragments you should be able to open this mail.

Thanks and have a great day!