You need to remove the domain from the URL, the format for the old URL needs to be like this:
/myolddepartment/bikes/whitebicycle.html' in your example it should be: ‘
I have tried to insert the URL with the right format and I got the same message.
The issue comes because you have a content page called: “Black Walnut Butcher Block” and even if for this page you have customized the URL the system still give you the possibility to go back to the non customized URL that in this case is:
If you did so, there would be a collision between the URL you are trying to introduce and the default URL, so the system doesn’t allow it.
You could solve it adding the content id to the defaults URL: