I’m in the process of setting up field mappings to sync the HubSpot lifecycle stage directly into Mailchimp’s “Tags” field. I want “Subscriber” or “Customer” to sync respectively as tags in Mailchimp. However, from this screenshot, you can see this is not possible. “Tags” is greyed out. Why would this not be possible?

Tags have to be created beforehand in Mailchimp through its user interface, so a direct field mapping won’t work here.
Since HubSpot does not work with tags, a field mapping to Mailchimp tags will not work either.
The PieSync license in which this screenshot is captured probably doesn’t allow for this type of mapping.

The correct answer is:

Tags have to be created beforehand in Mailchimp through its user interface, so a direct field mapping won’t work here.