iPhone not connecting on new device (MacBook Air)

Hi, I've just upgraded to a new MacBook Air from an older one. The iPhone can still connect to rekordbox on the old MacBook Air, but not the new one. I can see that the MacBook Air is seeing the iPhone, but when I click 'connect', the message 'Can't Connect To iPhone' results.  Apparently, if I'm not mistaken, we can register up to three devices (I'm on the free plan) but I need to register the new device (I can't find out how to) and disconnect the old Mac (but I can't find out how to). Any pointers please. It's probably worth saying when I received the new MacBook I transferred all my files over from the old Mac, including Rekordbox - so I never installed a new version - and I've since deleted the imported version and installed a newer one, yet all the files/playlists/settings were somehow found and remain the same, and I can't register the 'new application'. I'm well and truly lost. Help...