For TIBCO targets, a non-header field cannot coexist with any of its direct/indirect parent groups in the TIBCO metadata tree.
When editing a TIBCO target definition, you wanted to add a group from the TIBCO metadata tree as a column in the target definition. You also wanted to add fields from the group as columns in the target definition. This is not allowed.
If you want to add a group to a target definition, do not add any fields from the group to the target definition.