Hi @Sian Melrose ,
Welcome to Q&A forum!
According to your description, Trello is a third-party software, and I have no way to test it. At present, there is no OOTB way to add Trello to SharePoint. If you are using a third-party tool, I suggest you ask the provider of the third-party tool for the reason for this situation. Or for this error, go to the Trello Communityto ask questions and get more targeted help.
Thanks for your understanding! :)
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