First, you have to find the content item created using this content type and delete them, and then you will be able to delete it.
If you can't able locate the content items then you can try -
Another option to find content is to go in Find -> Overview screen
You can find content based on a particular content type along with content ids. Search these content ids in CMS -> blocks section (right side window) or pages content tree.
Find with Ids and delete them. Clear Trashed content as well.
We created a solution to be able to tag our customer stories in the CMS backend and then allow users to filter them through the frontend.
We created the categories through code. I had to delete one of them i.e. delete the code.
Now in the EpiServer CMS in the Admin > Content Type I am unable to delete these content types. I see a small warning icon next to 2 of them that have been deleted from the code. But when I attempt to delete them I see the message: The content type could not be deleted since it is in use.
I have tried restarting the web app based on what I have read online, but this did not help.
I would be thankful for any other suggestions.