I kept geting "An unknown error occurred" when creating a remote on Bitbucket cloud using Xcode10.
Finally I figured it out.
"Repository Name" was causing problem. When you create a remote in Xcode 10, the default name is your project name in Xcode. But after several test, I found that this name can NOT include upper case letters or spaces. After I changed the name to all lower cases, and replaced spaces with "_", everything is back to normal. Interesting thing is I was able to create a repository on the website using upper case letters and spaces.
Xcode 10 should be able fix it because upper letter and space are allow on the web page.