Unable to delete a particular folder
It is always a one-step process to delete a folder from the navigation hierarchy in the usual scenario.
Right-click on the folderDelete
This usually happens when the folder is being references in another location, possibly a template.
In such cases, go to the template folder and check the source code of the template.
In below example, the source code of the template contains the folder name which means it is being referenced in this template and since the template is still present, it is preventing the deletion of this folder to avoid any further issue.
Folder Name: TAF
Source code of template where this is being reference:
Hence, you would need to review the templates and look for references and make required amendments.