When a hero is assigned for some role, such as a team member, the hero cannot be sold or used as a material. The current status of the hero can be checked with the icon at the bottom on the left by the hero image.

[Hero activity according to an icon]

1.  If a hero belongs to the main team 

2. If a hero is on the 2d deck team 

3. If a here is in the arena defense team or attack team 

4. In case when a hero is dispatched to an expedition 

5. If a hero is set as a profile main character 

6.  If a hero is locked 

If your hero status is not applicable to any of these situations, but it is still impossible to use or sell materials, please contact us 1:1 and attach the screenshot illustrating your issue.