404: Not FoundCouldn't find Transmission's web interface files!

Users: to tell Transmission where to look, set the TRANSMISSION_WEB_HOME environment variable to the folder where the web interface's index.html is located.

Package Builders: to set a custom default at compile time, #define PACKAGE_DATA_DIR in libtransmission/platform.c or tweak tr_getClutchDir() by hand.
On Linux Arch / Manjaro, trying to install package "transmission-cli": pamac install transmission-cli

-> that SOLVED it, i refresh that localhost page and interface is working.