By default, Iterations is disabled in a Workspace.
A Workspace Owner or Admin can enable Iterations by going to the settings gear wheel and then Workspace Features > Iterations.
Once Iterations is enabled in a Workspace, all users in that Workspace will see the Iterations icon in the navigation bar on the left. Users can start creating and using Iterations by clicking on the Iterations icon.
The Iterations icon (and other Iterations UI) will not show up in a Workspace if Iterations is disabled.