GraphQL: Query.type field type must be Output Type but got: undefined

For me, the problem was that I defined a graphql type and input type with the same name in my graphql schema file.

I changed this:

type InternalAttribute {
    name: String
}

input InternalAttribute {
    name: String
}

to this:

type InternalAttribute {
    name: String
}

input InternalAttributeInput {
    name: String
}

inside your schema.js as you create Object Type for String as

 fields: {
              name: {type: String} 
    }

same is needed for your custom fields too therefore

fields: {Companies}

Inside your Query type definition, you wrote:

fields: Companies

It should be

fields: { Companies }

Looking at the docs, fields takes an object or a function returning an object, with that object's keys mapping to the names of all the fields in the type (all the queries in your schema in this case).

Also, for clarity, since queries and mutations are fields of their respective types, it may be better to name them in camelCase rather than PascalCase (i.e. { companies: Companies }