Since you would be publishing first time to Adobe Captivate Prime from Presenter, there will be no entries for your Prime account.
You should click on Publish button and publish your project to you Prime account.
From next time onwards, you will directly be able to select the Prime account from the drop down,
Adobe Presenter Engineering Team
I am experiencing the exact same issue.
I am using a trial version of Presenter 11.1 with PowerPoint 2016. I am having the same problem with with no accounts listed in the pull-down, and the problem that the OK button stays grey if I Publish.
I can connect and log in using the Adobe Connect option, but the OK button is grey as well. I can upload static files, but not interactive. Zip files will not upload.
Not sure if you were able to fix the issue, but I tried it several months ago and got this working. Try the steps below:
Open registry on your computer and got the location below
Create a new string with a name StartURL and add the value as https://captivateprime.adobe.com/inappstarter.html