Doi unassigned even though plugin is rightly configurated


We’ve upgraded the version to OJS a couple of days ago and, even though the DOI Plugin is enable and setted up, now this message appears

I enter in any submission in Production and the click in Schedule for Publication, the message in image above appears. Ive tried all I know and I sincerely can’t seem to find a solution. Can anyone enlighten me?

Thanks a lot