CraftCMS Gatsby project throwing error "GraphQL Error Expected type [String], found {eq: $slug}."

I just started working with Gatsby to see if it would be a good choice to rebuild my company's CraftCMS website with Craft as the backend and Gatsby as the frontend. So far everything has been working well until it came time to query for the individual entries inside our "campaign" channel.

For the record, I have been able to render a complete list using .map() for each of my campaign entries on a "overall view" page to see all the campaigns. I have also been able to recursively build out each campaign page so that it calls my /src/templates/campaign-page.js template and has the correct slug pulled from my site's Craft API with no issue. For some reason, I just can't get my individual campaign data to query inside the campaign-page.js template.

I've read just about every page in the Gatsby docs and every tutorial that currently exists, but for the life of me I can't figure out why my GraphQL query will not filter for my individual campaign entries. It just keeps telling me, "GraphQL Error Expected type [String], found {eq: $slug}."

I've also tried wrapping my "slug: {eq: $slug} in a "filter:" based on some markdown docs, but that just tells me "filter" does not exist. I'm beginning to think the issue is in my gatsby-node.js file, but I'm not seeing any issue when I compare it to the docs.

Gatsby-node.js

const path = require(`path`)
exports.createPages = async ({ actions, graphql }) => {
  const { data } = await graphql(`
    query {
        api {
            entries(section: "campaigns") {
                slug
            }
        }
    }
  `)

  data.api.entries.forEach(({ slug }) => {
    actions.createPage({
      path: "/campaigns/" + slug,
      component: path.resolve(`./src/templates/campaign-page.js`),
      context: {
        slug: slug,
      },
    })
  })
}

Campaign-page.js

    export default ({data}) => {
        const post = data.api.entries
        return(
            <div className={"campaign-page-single"} style={{marginTop: '-21px,'}}>
            <Header/>
                <div>
                    <h1>{post.id}</h1>
                </div>
            </div>
        )
    }

    export const campaignQuery = graphql`
        query ($slug: String!){
            api{
                entries (slug: { eq: $slug }){
                    slug
                    id
                    ... on API_campaigns_campaigns_Entry {
                        id
                        campaignTitle
                        slug
                    }
                }
            }
        }
    `

For reference, here's what a typical working query looks like on my main campaigns.js page that lists all available campaigns:

           query = {graphql`
                {
                    api {
                        entries(section: "campaigns") {
                            ... on API_campaigns_campaigns_Entry {
                                id
                                campaignTitle
                                uri
                                slug

                            }
                        }
                    }
                }
            `}

I expect my /src/templates/campaign-page.js template to render the individual campaign data.