A 400 error is a "bad request" or "not found" error. I'd verify that your Trello connector is properly working, by going to the Data > Connections area of Flow: 
Then on the Trello connector click the ellipsis to select Delete and/or Switch account to re-enter your login credentials for Trello:
Also, on the Trello steps within your Flow, insure that you are utilizing the correct connection, via the My connections popup from the ellipsis:
Again, this may be an access or authetnication issue, so insure that on the Trello side everything is working as intended and the account has full read/write access to the Trello boards.
If that doesn't work, if you could provide an expanded screenshot of your Flow and steps, and of any detailed error messages you're receiving we could likely better assist you.
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