Download source error

I am trying to download the source on another machine.
If I follow:
git clone git@github.com:FreeRADIUS/freeradius-server.git
cd freeradius-server
git fetch origin v3.1.x:v3.1.x
git checkout v3.1.x

I get error:
$ git fetch origin v3.1.x:v3.1.x
fatal: Couldn't find remote ref v3.1.x

If I try into a new directory
git clone git@github.com:FreeRADIUS/freeradius-server.git
cd freeradius-server
git fetch origin v3.0.x:v3.0.x
git checkout v3.0.x

I get
$ git fetch origin v3.0.x:v3.0.x
>From github.com:FreeRADIUS/freeradius-server
 * [new branch]      v3.0.x     -> v3.0.x

User@User-Zoo MINGW64 ~/Documents/GitHub/freeradius-server (v4.0.x)
$ git checkout v3.0.x
error: The following untracked working tree files would be overwritten by checkout:
        .gitattributes
        .gitignore
        .travis.yml
        COPYRIGHT
        CREDITS
....
        suse/radiusd-pam
        suse/rcradiusd
Please move or remove them before you can switch branches.
Help please