This is a common issue for folks - Drag and Drop does not work for them when adding drills to practices, players to drills, and players to groups.
The two main solutions are:
1) Ensure you touch the item to be dragged long enough for it to 'pop' up under your finger, then drag to the indicated 'drop area'
2) If the item will not 'pop', try rebooting the phone. iPhone 7's seem to have some issue with Drag and Drop with the first use and reboot has helped everyone who has sited this as an issue.
Please add comments if you're experiencing other issues, and file things to be fixed in the 'Issues' forum.