I just upgraded to Xcode 10. It has built-in bitbucket support. I am already using bitbucket. Always do "add existing remote" after create a repository on the website. So no problem so far. With Xcode 10, after I added my bitbucket account in Xcode preference, I tried "create (my app name) remote ..." for a test project. But I got "An unknown error occurred". Tried both HTTPS and SSH, still not working.
Finally I figured it out by myself!
"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.
Edit:
Found the official reason:
The name is used as "slug".
The error message is:
Invalid slug. Slugs must be lowercase, alphanumerical, and may also contain underscores, dashes, or dots.
Xcode should have shown this to the developer to save them hours of frustrating.