I'm not sure I understand what you're trying to do.
Is code in improbable-marco trying to connect to your machine? From the error it looks like you're trying to use localhost:3000 as the address, but that won't work if the code is running in Glitch - localhost will be the Glitch server.
In general for this kind of error, you would add CORS headers, but I'm not sure if that would help, or if it would be safe security-wise without knowing more about what you're trying to do. Something like this will add the headers:
res.header("Access-Control-Allow-Methods", "GET, PATCH, POST, PUT, DELETE, OPTIONS")