Due to a long-term planned
of the addon repository,
the installation of addons through
g.extension is currently not working
on UNIX-like systems.
Installation of addons with the current stable version GRASS GIS 7.8.5
on OSGeo4W (Version 1)
is not affected.
All the necessary changes in g.extension have been implemented, merged and back-ported. The functionality will be back to normal very soon with the upcoming GRASS 7.8.6 and 8.0.0 releases.
In the meantime, you have two options:
master, where g.extension changes have been applied.
For the case of python addons, just download and run them as regular python scripts.
Thanks for your patience!