I'm trying to load an SSH key to Github from a new machine and this command is not working:

sudo apt-get install xclip

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package xclip is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package xclip has no installation candidate

when I try:

sudo aptitude install xclip

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Reading extended state information      
Initializing package states... Done
No candidate version found for xclip
No candidate version found for xclip
The following partially installed packages will be configured:
  synaptics-dkms 
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B of archives. After unpacking 0B will be used.
Writing extended state information... Done
Setting up synaptics-dkms (1.1.1) ...
Loading new synaptics-1.1.1 DKMS files...

Error! Cannot locate /usr/src/synaptics-1.1.1.dkms.tar.gz.
File does not exist.
dpkg: error processing synaptics-dkms (--configure):
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 synaptics-dkms
E: Sub-process /usr/bin/dpkg returned an error code (1)
A package failed to install.  Trying to recover:
Setting up synaptics-dkms (1.1.1) ...
Loading new synaptics-1.1.1 DKMS files...

Error! Cannot locate /usr/src/synaptics-1.1.1.dkms.tar.gz.
File does not exist.
dpkg: error processing synaptics-dkms (--configure):
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 synaptics-dkms
Reading package lists... Done             
Building dependency tree       
Reading state information... Done
Reading extended state information      
Initializing package states... Done

Any idea how I can install this?

Mucho thanks in advance