I can create a pull request to my repository (let's call it myrepo) from BitBucket and it goes to this URL and works fine:
https://bitbucket.org/my_company/my_repo/pull-requests/new?source=my_company/my_repo%3A%3Afeature/my_feature_branch
From within SourceTree using the same repository I go to create a pull request and it jumps to this URL in the browser and displays"That link has no power here":
https://bitbucket.org/my_company/my_repo/my_company/my_repo/pull-request/new?source=my_company/my_repo:70dec24473f5:feature%2fmy_feature_branch
So notice that my_company/my_repo appears twice and also pull-requests is used instead of pull_request (notice the extra 's').
It looks to me like the SourceTree URL is very different and thus will not work.
In the repository settings in SourceTree, all looks fine:
URL/Path is git@bitbucket.org:my_company/my_repo.git
Host Type is BitBucket
Host Root URL is https://bitbucket.org/my_company/my_repo
SourceTree Version 2.6.6.0