How do I connect the fields with the different facilities?

On the “Forecast wells” tab for Forecast properties, you have a choice to select a Field each well is connected to. If it is greyed out, most probably the well is a producer, and then this settings is already made on the “Production potential” tab where you can set the field that it is connected to. Going back to the “Forecast well” tab, you can define a facility and a cluster for the well. But you may only choose clusters which are defined within your selected facility.

The cluster you choose should be a cluster for the facility you have chosen, regardless of the field. You can have several facilities within the same field, and many cluster within each facility, but you have a limit of number of clusters levels you can have in your hierarchy, maximum seven levels.