I can't assign a blueprint

Hi, I have started a site with Plainkit of Kirby 3. I have created a blueprint for a specific page (work.yml) that is a subpage of Obras. But I have two problems:

  1. When I change the name of the .txt to work.txt to use the work.yml blueprint in the subfolder of Obras, the page not appears in the section panel “Obras”.
  2. If I try to assign via panel a blueprint to a page that I have created in Obras, the option to do that is deactivated.

My blueprint:

Page title: Work

columns: 
  - width: 1/2
    sections: 
      work-info: 
        type: fields
        fields: 
          year:
            label: Año de producción 
            type: number
            width: 1/2
          dimensions: 
            label: Dimensiones
            type: text
            width: 1/2
          technique:
            type: text 

  - width: 1/2
    sections:
      fields:
        type: fields
        fields:
          mainimage:
            label: Portada
            type: files
            layout: cards
          details:
            headline: Details
            type: files
          work_video:
            label: Video
            type: embed

Thanks,

I have created a blueprint to mainfolder/mainsection Obras and a template to work, a work.php. Now it works.