Can't get thumbnail in the blog

[WatheqAlshowaiter]

I want to get thumbnail in each blog post.. and I am using Netlify CMS

I get all the time the error message: Field "thumbnail" must not have a selection since type "String" has no subfields.

My query is

    thumbnail {
          childImageSharp {
            fluid(maxWidth: 800) {
              ...GatsbyImageSharpFluid
            }
          }
        }

what can I do to make it work well?

[thundermiracle]

This starter is not using Netlify CMS.
If the issue is not concerned to the repository, I think you'd better ask it in GatsbyJS or StackOverflow. Or search other starters like this:
https://www.gatsbyjs.com/starters/netlify-templates/gatsby-starter-netlify-cms