Here's a reference setup of AWS Lambda and Serverless and GraphQL that i'm following:
https://github.com/serverless/serverless-graphql-apollo
I'm trying with yarn run start-server-lambda:offline to start the project offline, it does start without any problems, but upon navigating to /graphiql, I get this:

serverless.ymland possibly the part of your handler where you setgraphqlendpoint thatgraphiqluses?