If you are adding a new unit, but can't remeber the HA password, you can always log into the primary unit and change this value to something which you know.
I would also suggest that having successfully migrated the primary's configuration from the F-series to S-Series appliance, it may also be easier to simply delete the secondary from the HA configuration and re-create it. Then you will know all of the details that you need to enter onto the secondary in order to get it to join-up.
The other key thing to remember with HA environments is the zone/burb entries on the secondary appliance must be created in the same order as they were created on the primary appliance. Either connect to the CLI on the primary appliance and run the "region" command, or look at each burb/zone in the GUI. You should find the "ID" value located just above the check boxes:-
Hope that helps you.
I have tried all the steps you gave. Also I tried the steps from the link below.
but still I cannot register the secondary firewall to the primary. I deleted the secondary and tried to change the primary to standalon but an error appeared
Error: TSWAttributeError: 'NoneType' object has no attribute'split' I tried to create a new secondary firewall to the cluster but still the secondary firewall cannot download the configuration files from the primary.
The error "Error: TSWAttributeError: 'NoneType' object has no attribute'split'" that you were getting when trying to change the primary to standalone is likely due to a misconfigured or partially configured interface on that firewall. Check all of its interfaces for any that are configured but not used/disabled. Can those be deleted? Or perhaps an interface is configured but not assigned to a NIC. Then try to demote the primary to standalone once again.