You cannot use the Rich-Text Editor, JSON Rich Text Editor, Markdown, File, Link, Custom, Taxonomy fields as the operand fields.
You can use the Group parent as an operand only when the Group field is marked as Single.
The target field cannot be a sub-field of a Modular Block, Group, or Global field marked as Multiple, unless both the operand and target fields belong to the same block, group, or global field.
If a field is marked as Mandatory, has validation applied, or is set to Multiple, placing that field inside a container such as a Group or Modular Block does not bypass the restriction.