files.completeUploadExternal returning channel_not_found in a raw curl upload flow: the channel ID you passed is wrong or the app cannot see the channel. Verify the channel ID is correct, and make sure the bot or app is invited to (a member of) the channel. Also, never paste real tokens into issue logs; regenerate any credentials you posted publicly.

_Source: gh:slackapi/python-slack-sdk#1520 (Slack API)_