You got into a loop inside the system, i.e. creating a new process based on this BP, a subtask is generated in the same BP, and at the starting stage of the subprocess, you also need to generate a subprocess in this BP, and so on ad infinitum
http://i.imgur.com/VldzHKh.png . The system sees the loop and prevents you from creating this process. It is necessary to remove automation from the first stage or reconfigure the action to create a subprocess (subtask) on some other BP