After a break i'm back on the Fortigate!
from the cli you saw above, i simply gave a delete "internal7" and the problem was solved!
now i have internal7 free !!!!!!!!!!!
Thank you all as always, i hope this is useful to others users.
example to delete the internal6 from CLI:
FWF60 # config system virtual-switch
FWF60 (virtual-switch) # edit "internal"
FWF60 (internal) # set physical-switch "sw0"
FWF60 (internal) # config port
FWF60 (port) # show
FWF60 (port) # delete "internal6"
FWF60 (port) # end
FWF60 (internal) # end