Hi @Aaron Manill ,
Welcome to Microsoft Q&A! Thanks for posting the question.
I see that you are trying to use PubSub in an existing project, however the extension bundle version, being used in this project, does not contain PubSub. Please note that - you can't explicitly install extensions in a function app that is using extension bundles. You will have to remove the extensionBundle section in host.json before explicitly installing extensions. Please refer to the Important section of this link: Explicitly install extensions.
--updated 03/08/2022--
The table documented here points to 3.4.0 version of extension.json
which has pubsub binding included in the extension bundle. However, this version has not been released yet. I am reaching out to the Internal team for more clarification and update as required.
Please let me know if you have any questions.
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