Try with a console set the enable password in addition, please create your user in privilege 15 modeyou can Set the telnet and SSH passwordHope you will get a result.Thanks 

Thanks I have got the solution form google as below  and working fine :Log into the appliance via SSH using the admin account.• For the Firepower Management Center, by default this gives you access to the shell. If the Firepower Management Center CLI...

How to change the admin password from CLI mode of a FTD and what is the save command from CLI