I have added the SSH key in Acquia Cloud, but I am not able to SSH in.
Log in Acquia Cloud and go to Manage the Organization.
Check the Role that is assigned to the user having issue to SSH in.
By default Developer Role do not has permission to SSH in, just Senior Developer and Team Lead.
To resolve this permission issue you could edit Developer role or create a new role.
To give SSH access to a role you will need to enable the following permission under Administration and SSH keys:
Administration
SSH keys
You don't need enabled all these SSH permission, just the one you feel more appropriated.
Understanding the the SSH key is added in Acquia Cloud, this could be related to the Role permission in the Organization.
You can see more information about the organization and how to create roles and give permission on https://docs.acquia.com/cloud-platform/access/teams/roles/.