I know there are some very similar questions regarding this topic: GRASS GIS addon "extension not found"; could the problem be with the GRASS Addons server?
but so far nobody has found a proper solution it seems. I am trying to install r.stream.distance (and other) addons but not with Grass 7.6 or 7.8 i get any success. I tried to run a different g.extensions script (as suggested in the question mentioned above) and the addon seems to install but then there are alwas problems in running the addon, specifically: