When you create a new developer Org with Einstein Analytics enabled, you won’t be allowed to create a new dataset initially because a user will need to have the “Edit Analytics Dataflows“ user permission and the organization will need Data Sync and Connections enabled in Analytics settings.

Use the below steps to enable it.1. From Setup, enter Analytics in the Quick Find box, then select Settings.2. Select Enable Data Sync and Connections, and then click Save. Now go to the data manager and check that you can create dataflow.