Installation - svn error

1
When I try to install psychtoolbox an error appears:

Sorry, the download command "CHECKOUT" failed with error code 1:
'svn' is not recognized as an internal or external command,
operable program or batch file.
...
Error using DownloadPsychtoolbox (line 775)
Download failed.


I've tried to install psychtoolbox on different PC's (with OS Win8 and Win10) using different Matlab versions (2013b & 2015b) but the result is always this same error.

I've installed SlikSubversion and the svn.exe path is shown among environment variables. Two months ago I successfully installed psychtoolbox with the same Matlab versions on identical PC's so this error now is even more confusing.

Any ideas on this matter?

Thank you in advance.

2
Whatever it is, "help DownloadPsychtoolbox" gives instructions how to deal with not finding the svn.exe executable in the systems search path.
-mario
3
Thank you, Mario. I've tried everything from the help DownloadPsychtoolbox, but the error won't go away. Yesterday I got somehow different error (the problem is the same, again it's the svn file).

'svn: E175009: The XML response contains invalid XML
svn: E130003: Malformed XML: no element found
...
Error using DownloadPsychtoolbox (line 775)
Download failed.'

No matter how many times I run the installation, this error keeps showing. When I clean and reinstall everything from the zero, this error and the error from my first post sometimes interchange.

Also, when I try to visit github page that Psychtoolbox obviously is trying to access:
Target folder for installation: d:\toolbox
The following CHECKOUT command asks the Subversion client to
download the Psychtoolbox:
 d:\Subversion\svn checkout  https://github.com/Psychtoolbox-3/Psychtoolbox-3/branches/beta/Psychtoolbox/ "d:\toolbox\Psychtoolbox"
Downloading.'
I receive 'Sorry, but this feature has been retired.' message.. Is Matlab trying to access the page that is out of use?
4
Thank you, Mario.
I manually downloaded Psychtoolbox now because I'm really in a hurry with my project.
I'll keep trying to find the solution for these errors and then let you all know what happened, maybe the solution is really trivial one.
Once again, thank you for the help.