I had the same problem and solved it by running grafana locally. Not sure why it matters but it doesn’t work when I use the main website login. I had to download grafana and run it locally then login through running the web server.
see this guide
to setup the datasource but note step 2 which says “download and install Grafana”
that takes you to the download page
for an easier set of instructions to set it up see link in step 3 “start Grafana”