Disable Driver Signature Enforcement in Windows before installing Xperia drivers.
Move your downloaded .ftf firmware file into the C:\Users\YourUsername\.flashTool\firmwares directory. Official Sony Xperia XZ SO-01J -Docomo Japan- Stock Rom
: It introduced the "Loop Surface" design with a body made of Official Sony Xperia XZ SO-01J -Docomo Japan- Stock Rom