Why eval `ssh-agent` may not work while setting up the SSH keys on Ubuntu?

[Volodymyr Dzhulai]

While setting up an SSH key due to https://support.atlassian.com/bitbucket-cloud/docs/set-up-an-ssh-key/ at the Step 2. Add the key to the ssh-agent for macOS/Linux I've discovered an error "Could not open a connection" after using the command eval `ssh-agent`. The problem was solved with the use of such a form eval $("ssh-agent") instead.  Why it has appered to me? And could you please add this specific syntactically different form of the same command to your description?