I know this is an old thread, but for the record I managed to get the Flux datasource working on InfluxDB 1.8.3 with Grafana 7.3.6. Step 1 was to enable the flux api in influxdb by adding
flux-enabled = true
to influxdb.conf under [http].
Step 2: Add a new data source, using the previous URL, but with adding "" to the organization field (yes, the two quote marks are needed, not just leaving it empty) and myuser:mypassword into the Token field.
Now I just need to figure out the new query syntax…
Sources for the instructions: