To activate/renew Power of the Sun Scroll, you need to meet the minimum sun scroll requirement. To find the minimum requirement, just hover over the fire next to Sun Scrolls.

As you can see above, I have to obtain 5000 Sun Scrolls this week to activate it. You can obtain the Sun Scrolls from doing the following events:
Rescue the Jinchuriki Sage World Battlefields
Nine Tails Cat Quiz
Fighting Matsuri Rogue Ninja Outburst
Matsuri Challenge Arena Training Grounds
It's like activating/renewing the Power of the Moon Scroll. Also, you can activate/renew for a total of two times this month so good luck.