This is the link to the github repo for my code.
https://github.com/Messan93/Messan93-WeatherJournal.io
Everything seem to be fine but these are the errors am still getting. I can see what the error message is but I don't see anything wrong with the code on that line.
You're using relative URLs on those API calls you're making, and since you're opening your index.html directly from your file system rather than serving it through a server, the paths are relative to your file system.
The simplest fix (just to get your example working) would be to provide absolute URLs instead:
Change postData('/add' to postData('http://localhost:3000/add'
and await fetch('/all') to await fetch('http://localhost:3000/all')
A more real-world fix would be to serve the index.html file from your express server.