What looks to be the issue here is that you’re using a version of Octo.exe that’s designed for later versions of Octopus Deploy. With Octopus version 2018.8.9, you’ll want to use version 4.41.5 of Octo.exe - you can download that version from https://octopus.com/downloads/2018.8.9 (which has all the version-correct addons and tools for 2018.8.9).
This should solve your problems, but if it doesn’t, please let me know and I’ll do my best to help you further.