Hi @domrue,
I was having a similar issue earlier today, have you tried using the id for the team rather than it's name?
If I POST to:
http://localhost:8065/api/v3/teams/development/channels/8yce4jb34irc9mrkjcnj7k8p1a/posts/create{"channel_id": "8yce4jb34irc9mrkjcnj7k8p1a", "message": "Message from API!"}
I get the same error you are reporting, however if I try:
http://localhost:8065/api/v3/teams/t8wxrxbwwtbwfbuteahkigunry/channels/8yce4jb34irc9mrkjcnj7k8p1a/posts/create{"channel_id": "8yce4jb34irc9mrkjcnj7k8p1a", "message": "Message from API!"}
I get the expected response, and my message is posted to the channel on my Mattermost server.
As a side note, I've noticed that using the id or name of the team when returning a channel list give different results.
http://localhost:8065/api/v3/teams/t8wxrxbwwtbwfbuteahkigunry/channels/ is returning all expected channels (channels, private groups and direct messages).
While http://localhost:8065/api/v3/teams/development/channels/ is only returning a list of channels for direct messages.
Hopefully this helps, I've only been looking at the API for a couple of days but happy to help where I can.