have you made the notebook public? unlisted is fine.
instead of calling .json() where the response is failing to parse as JSON, you can call .text() to debug.
I beleive it is printing out the error message found on this line webcode.run/index.mjs at main · endpointservices/webcode.run · GitHub
which is Deployment '${req.requestConfig.name}' not found, did you remember to publish your notebook, or is your deploy function slow?
i.e. your notebook is not publically accessible (requirement for fetchp and all webcode serverless endpoints unless you are a PRO customer)