Wasted an entire day trying to sort this crap out and was finally able to resolve the problem by following an article on Medium. This should never be the case. The official documentation should always be whole and sufficient.
The specific doc I'm referring to is: https://confluence.atlassian.com/bitbucket/set-up-an-ssh-key-728138079.html
It does not take into account having multiple Bitbucket accounts, and it doesn't even mention specifying a minimum key size of 2048 when generating one using `ssh-keygen`; this is required by Bitbucket and you get an error if you add a smaller key.
Either update the doc or make the documentation open-source.