I have downloaded and installed flashtool (program) I should do reboot into fastboot mode (via fastboot)
This is basics and has nothing to do with this thread But ok, open your explorer inside Windows and navigate to 'C:\Flashtool\drivers', then double-click onto 'Flashtool-drivers.exe', then check the checkboxes of Flashmode Drivers, Fastboot Drivers, Common drivers Rhine board and Xperia Z1 driver.
Btw., if you are on Windows 10, then you have to deactivate the signature check (just google for it!) in Windows 10 before the aforementioned. Otherwise it won't install all drivers correctly
But now btt...