This classification node cannot be deleted because it contains article group classifications.
Explanation:
The own classification consists of a maximum of 4 layers. If you delete a 4th classification layer which still has articles classified under it, an error message is shown on the screen.
In other words, deleting a classification layer on the 4th level is only possible if all the underlying articles have first been placed in other classifications.
Solution:
First classify all underlying articles in another classification and only then remove the 4th classification level.
The example below shows the same action, but with a 3rd level example.
Wrong: Under the 3rd level classification layer "Codes" is a fourth layer: "Codes" present
Right: The 4th classification layer "Codes" has been removed first, after which the third layer, classfication layer "Codes" can be removed.
