Hero Assemble does not allow the creation of duplicate nicknames. 

Please set a non-duplicate nickname, 
If you want to change your nickname in the future, please refer to the following instructions. 

■ How to change the nickname
1. click the [Profile] menu on the top left of the screen → tap the [🖊] pencil-shaped icon to the right of the nickname
2. click the [≡] menu on the top right of the screen → tap [⚙] Settings → click [Change account ID].