If you have a failed backup that shows an output message of "s3: command not found" then somehow the awscli package was not installed. You can install it manually by following these steps:

First we want to make sure you have python and PIP installed and at acceptable versions. You can check this with the following commands:

python --version (we want to see >= 2.7)

pip --version (we want to see >= 7.xx)

Given that these components are at the right versions we need to do is run the command sudo pip install awscli. Once that is done you should be able to run which aws and verify that it is installed. Subsequently running aws --version will tell you the version (we want
>= 1.8.3) and that it runs without errors.
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If you get to aws --version ok you should be able to do a backup.