Update the package list first:
sudo apt-get update
Then check if the package is available:
apt-cache search sshpass
If that succeeds, installing should be possible.
If not, check your repositories. The list is stored in the file
/etc/apt/sources.list and within files at
/etc/apt/sources.list.d/.
To add repositories use
sudo add-apt-repository <repo>
The default repositories are
main,
restricted,
universe, and
multiverse.
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