Hey friends - I’m not sure if I am supposed to write advice on here but I just discovered a WONDERFUL tool: http://www.yamllint.com/
Since yaml files are so finnicky about whitespace, I have run into many errors trying to run trellis commands from the command line such as:
yaml: line 9: mapping values are not allowed in this context
yaml: line 12: did not find expected key
Needless to say this has been an annoyance for me since the error does not specify the file name and I had just edited a couple of files in group_vars.
So next time you run into a trellis error: STAY CALM & USE THE YAML LINTER 
Turns out I was missing just one space in the wordpress_sites.yml file …
Hopefully this can help someone today or in the future.
Cheers