Hello, this log you mention related with GRASS GIS version 8.6.0dev (main GIT branch). Issue especially related with this PR.
Extension installed locally successfully work with g.extension module. Problem is related with compilation (GRASS GIS switched from usage Make to CMake for compilation) addons via this script file https://github.com/OSGeo/grass-addons/blob/14f2ab1c765381dd9b1c949e6a65cb8347f66662/utils/cronjobs_osgeo_lxd/compile_addons_git.sh#L155-L164 (make command is still here used) on the server.
See my initial comment here.