Your Panoramio gallery seems to be already connected to G+. So that's why you are getting an error if you try to connect it again.
My guess: Photos from Panoramio gallery will be transferred to
Google Album Archive at the very end of Panoramio existence (November 2017).
Possible reason? Panoramio users can still delete photos from their Panoramio galleries... and it makes a sense to transfer only the "final version" of their galleries. Nobody has Panoramio photos transferred so far (as I know).
I also read the message from Google that all photos from Panoramio will be saved to Album Archive, regardless if you link your Panoramio gallery with G+ account or not, but these photos will be available only for you... (no public exposure, no sharing options).
If you link Panoramio with G+, your photos remain visible & searchable publicly on Google Maps.
But if you want to keep the access to your photos, it is safer to keep Panoramio and G+ linked anyway.