how to build schema using TS?

[nakajimayoshi]

I'm going through the tutorial but implementing using TypeScript and tsx instead of Javascript. I'm stuck at the services/index.ts

import { request, gql, GraphQLClient } from 'graphql-request';

const graphqlAPI = process.env.GRAPHCMS_SECRET;

export const getPosts = async () => {

    const query = gql `
        query MyQuery {
            postsConnection {
                edges {
                    node {
                        author {
                            bio
                            name
                            id
                            image {
                                url
                            }
                        }
                        createdAt
                        slug
                        title
                        excerpt
                        featuredImage {
                            url
                        }
                        categories {
                            name
                            slug
                        }
                    }
                }
            }
        }
    `
    const result = await request(graphqlAPI, query);
    return result.postConnection.edges;
}

All of the fields return this error:
Unknown field "postsConnection": The parent selection or operation does not resolve to a valid schema type

and the graphqlAPI param in the return throws this error:
S2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.   Type 'undefined' is not assignable to type 'string'.

I don't know where to assign the string datatype or how to make this schema valid, it's exactly like the one in the video but this one throws error for some reason