Your Scrum board has the following filter query:project in (ENG.

The reason why you are unable to drag an issue into a future sprint in the backlog is that ranking is disabled. Ranking is a feature that allows you to order issues based on their relative priority or importance. Ranking is enabled by adding “ORDER BY Rank ASC” or “ORDER BY Rank DESC” to your board filter query. Ranking is required for Scrum boards, as it determines the order of issues in your backlog and sprints. If ranking is disabled, you cannot drag and drop issues in your backlog or plan future sprints. References: Ranking an issue, Configuring filters