I've just found that the Designer & Photo persona buttons are still disabled for me in the RC1 build. Publisher is still throwing up the message "This feature is disabled until later in the Publisher Beta".
Figuring this was a little weird for a release candidate, I got on the forums and found personas should be working since 1.7 of each app.
Since Designer and Photo are both installed in their full 1.7 release versions, I completely removed (including all prefs) and reinstalled Publisher 1.7.0.384 to see if this helped, but no difference. Personas still disabled.
Both Designer and Photo have been purchased through the Mac app store; would this be the source of the problem?
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MacOS 10.14.5
Publisher 1.7.0.384
Designer 1.7.0 (from Mac app store)
Photo 1.7.0 (from Mac app store)
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