g.extension problem on Linux

I run Grass 7.8.4 on Ubuntu 18.04

I cannot install extension with g.extension. Follow the message from the 
console :

g.extension --verbose extension=r.stream.basins
Fetching <r.stream.basins> from GRASS GIS Addons repository (be patient)...
Downloading source code for <r.stream.basins> from 
<https://github.com/OSGeo/grass-addons/trunk/grass7/raster/r.stream.basins> 
which is identified as 'official' type of source...
Traceback (most recent call last):
   File "/usr/lib/grass78/scripts/g.extension", line 2183, in
<module>
     sys.exit(main())
   File "/usr/lib/grass78/scripts/g.extension", line 2163, in
main
     install_extension(source=source, url=url, xmlurl=xmlurl)
   File "/usr/lib/grass78/scripts/g.extension", line 765, in
install_extension
     source=source, url=url)
   File "/usr/lib/grass78/scripts/g.extension", line 1406, in
install_extension_std_platforms
     outdev=outdev, directory=srcdir, tmpdir=TMPDIR)
   File "/usr/lib/grass78/scripts/g.extension", line 1346, in
download_source_code
     download_source_code_official_github(url, name, outdev,
directory)
   File "/usr/lib/grass78/scripts/g.extension", line 1229, in
download_source_code_official_github
     url, directory], stdout=outdev) != 0:
   File "/usr/lib/grass78/etc/python/grass/script/core.py",
line 74, in call
     return Popen(*args, **kwargs).wait()
   File "/usr/lib/grass78/etc/python/grass/script/core.py",
line 63, in __init__
     subprocess.Popen.__init__(self, args, **kwargs)
   File "/usr/lib/python3.6/subprocess.py", line 729, in
__init__
     restore_signals, start_new_session)
   File "/usr/lib/python3.6/subprocess.py", line 1364, in
_execute_child
     raise child_exception_type(errno_num, err_msg,
err_filename)
FileNotFoundError: [Errno 2] No such file or directory:
'svn': 'svn'
l. 1344 url : 
https://github.com/OSGeo/grass-addons/trunk/grass7/raster/r.stream.basins 
name : r.stream.basins directory : 
/tmp/grass7-rlagace-24713/tmp1htzjxu5/r.stream.basins
l. 1228  url : 
https://github.com/OSGeo/grass-addons/trunk/grass7/raster/r.stream.basins 
directory :  /tmp/grass7-rlagace-24713/tmp1htzjxu5/r.stream.basins
(Tue Dec  1 16:03:04 2020) La commande s'est terminée (0 sec)

   The problem occours at lline 1227 with the command

if grass.call(['svn', 'export',
                    url, directory], stdout=outdev) != 0:

I had two print lines (l. 344 and l. 1228) to print variables to help debug

The name of the github is 
"https://github.com/OSGeo/grass-addons/tree/master/". I change it at 
line 2045 but without success.

What is the use of 'svn' in grass.call()

Thanks,

-- 
-----------
Robert Lagacé, ing., agr., prof.titulaire
Département de SGA
2425 de l'agriculture
Université Laval
Québec, Québec, G1V 0A6
courriel : [email protected]<mailto:[email protected]>
tél: (418)-656-2131 poste 402276
fax : (418)-656-3723

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