How can I install FlashTool in leap 15.4 ?
Please be more precise. Give a link to the tool you want to install. The forum search additionally throws this results:
https://forums.opensuse.org/search.php?searchid=9559453
I do not think there is no build available, but it seems easy enough the build it yourself: https://github.com/Androxyde/Flashtool/blob/0.9.34.0/BUILD.md
If you are going to your flash Sony phone, being able to build things is I think a prerequisite 