Note: This version is considered legacy. For modern Xperia devices, please use Newflasher or Emma by Sony.
You cannot use Flashtool without firmware files (ending in .ftf). You will need a tool like XperiFirm to download the official firmware for your specific device model. Once downloaded, move the .ftf file to the C:\Flashtool\firmwares folder.
While Flashtool has seen many updates over the years, version 0.9.18.6 (released around 2015) holds a specific place in the software's history.