The first reason can be that you have not set ssh keys. To do so, follow these steps. But step 4.9 for linux is not complete enough: you should expect ssh to tell you that the host is unknown etc. you can proceed safely.
The second reason can be caused on a per project basis, and it is because of the way you add your remote: you need to add it in the ssh format, not the https, like so:
git@bitbucket.org:myusername/myrepo.git
That's it.