What (if any) are you getting in Flashtool?
If you have downloaded the .ftf file and installed Flashtool, follow these steps to flash your device: Open the application. What (if any) are you getting in Flashtool
Ensure your Xperia XZ Premium has at least 50% battery capacity. Turn on your Xperia XZ Premium
Turn on your Xperia XZ Premium. The initial boot can take anywhere from 5 to 10 minutes as the system initializes the new stock software. Troubleshooting Common Flashing Errors If it happens, you have corrupted the modem partition
This is rare with a verified single SIM FTF. If it happens, you have corrupted the modem partition. Reflash again with the “BASEBAND” option selected in the exclude list (keep it included). If that fails, a full service repair with EFS backup is needed.
This utility is usually integrated into Flashtool or can be downloaded separately. It allows you to download verified, uncorrupted official FTF firmware directly from Sony’s servers.