Unable to add a Story to a future Sprint

[Mobily Monks]

We are using Next-gen projects in JIRA Cloud.

We are currently in Sprint 5  and want to add new Stories to a future sprint Sprint 6. 

We are trying to add these new Stories to a future Sprint as part of a advanced planning exercise.

Problem we are facing is follow

I am not able to add a Story(currently this Story is a child of an EPIC) to a future sprint, Sprint 6? 

When the same Story( is not child of an any EPIC), I am able to add it to a future sprint?