Cannot select a custom field in "Group results by" on the issue list of Redmine
The Group results by option of the issue list groups the issue list by the value of the field that you select.

Example: Issues grouped by assignee
Custom fields that you can select in Group results by
You can also select a custom field in Group results by. However, the custom field must meet all of the following four conditions.
- Format is one of the following
- Key/value list
- Version
- User
- List
- Integer
- Date
- Boolean
- Multiple values is turned off
- Visible is "to any users", or it is "to these roles only" and you are a member of a project with one of the specified roles
- The custom field is enabled in Projects (this does not apply to issue lists across projects, such as all projects or a parent project with its subprojects)

The page to create or edit a custom field
