Problem:
I wanted to add a taxonomy to the existing custom post type "Offers". When I selected the taxonomy in question and wanted to save the post type the following error message came up:
"You cannot use this slug because it has already been used or contains a reserved word. Please select another slug."
So I changed the previous slug "sport-offer" slug to "sport-offers", but now the offers do not appear. How can I restore the previous slug "sport-offer".
Solution:
By chance, have you already tried to check if there is any page/post/content template/view that is using the 'sport-offer' slug?

WordPress do not allow duplicate slugs, and therefore, they should be unique. If you haven't checked it yet, can you please try to find where you might be using the slug "sport-offer", and then rename that slug?

After this, it will allow you to create the taxonomy with the desired slug.