You can have MongoDB Cloud Manager and MongoDB Atlas groups associated with the same login.
If you see a "New Deployment" button rather than a "Build a New Cluster", you are looking at a MongoDB Cloud Manager group rather than a MongoDB Atlas group. The logo at the top left of your page should also indicate whether the currently selected group is Cloud Manager or Atlas.
To add a MongoDB Atlas group:
In Cloud Manager, click on your user name in the upper-right hand corner and select My Groups. Click the Add Group button.
The Create a New Group screen provides you the choice to create either an Atlas group or a Cloud Manager group. Select MongoDB Atlas to create a new Atlas group and switch to the Clusters view in Atlas. To add clusters to the new group, continue to Create a Cluster.
The above steps are from the MongoDB Atlas FAQ: As an existing MongoDB Cloud Manager account holder, how do I create a new MongoDB Atlas group?.