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You don't tell us which version of Raspbian you are using, but this is most likely a Python 2 vs Python 3 problem.

For typical platforms apt-get install python-yaml will install the Python 2 library.

From the shebang in your Python script:

#!/usr/bin/python3

you appear to run your script(s) using Python 3. As yaml is most likely not installed for that, the runtime can't find the module.

I suggest you switch to Python 2, as that is the default -- and best supported -- in all currently released ROS releases.