If you get enough herostones to summon a hero while completing the Tower, and try to summon them right there, an error message can appear.

This is a known issue, and are working on a fix. Meanwhile, we suggest you to refrain from summoning the hero until you finish your Tower run.

There is also a known issue that occurs if you open the Tower Shop and tap the cart icon, the Shop can show you that you have 0 Mystery Sapphires. If you tap them, and then tap the "Find" button to open the Tower again, the game will show you an error message.

We are also aware of this bug, and are fixing it. In the meantime we recommend you to open the Tower shop by tapping the "Shop" tab on the main game screen.