When the Sprint form is viewed, in default view, we do not see the Assignment Group field. However, it is normally seen in the Scrum view, which displays the Assignment Group field, as read-only.
Out of Box, a Dictionary Override on assignement_group for rm_sprint table has 'Override read only' and 'Read only' checked and set to true. Unchecking 'Override read only' and saving will make Assignment Group on Sprint writable.
1. Navigate to sys_dictionary.list
2. Filter for column_name = assignement_group
3. Open 'assignment_group'
4. In the Dictionary Override related list, select the override with rm_sprint as the table
5. Uncheck 'Override read only'