2013 - Can't edit the subsites Master Pages

Question

Situation :

On my SharePoint root site, I can modify without any problem my master pages. But from this root site, I've created few subsites. I would like to edit master pages from theses subsites in order to add them a registration for my custom CSS file.

Issue :

But, when I try to edit the subsites with SP Designer, it fails with this error message : This page is based on a file of the site definition, and you are not allowed to customize it. Please contact the site collection admin. (I translated from an other language message, so it's possible that is not the exact terms).

I checked the account used in SP Designer, it has the full control rights on the master page element.

But something is also weird: From a web browser, when I want to check permissions of the master page, I'm going to the subsite settings > masters pages > I select one, and click on "File" tab in the top ribbon, and then on the "Shared with" button. It fails, and display the following error message :

The column does not exist. It may have been deleted by another user.

So I don't get why I can't modify it ! Thanks a lot to anybody which could help me :)

Answer 1

I'm not sure if this addresses your specific problem, but this may help you work around it.

Try creating a copy of the root site's master page and adding your CSS reference there. Then go to the Design Manager and publish it. Then apply that Master page to the device channels, making sure to select the check-boxes here:

Answer 2

Go to Site Collection administration settings -> SharePoint Designer Settings. Made sure all the boxes are checked .