With the new UI, there is no longer any navigation for accessing a repository's admin settings page. There used to be a little gear icon, but that has been removed. If you create a new empty repo there is no way to access the settings page unless you manually enter the URL (e.g. bitbucket.org/username/repo-name/admin). Once you have code in the repo, there is a share button in the top right corner of the overview page and through that you can find your way to the access management page by going to the "Repository" tab and clicking "Manage this repository," which takes you to the access management page. This is very convoluted and not at all obvious, nor helpful when the repo is still empty.
There was a case recently where my colleague created a new empty repo and realised he had created it under his personal account instead of the company account and needed to transfer it before we could grant access to the right people. It took two of us 20 minutes to figure out how to simply access the settings page where the transfer option can be found.
Seems to me there should be a "Settings" link under "Navigation" on the left - that makes the most sense to me.