Hey @Monaye9! This type of error often occurs when you are using our personalization code15 and it has a syntax error in it. I would recommend trying to remove any personalization code you are using altogether, and then try re-adding it, and see if that solves the issue.
However if the issue persists after that, please reach out to our team at firstname.lastname@example.org (or via the in-app messenger in your account) with a description of this issue and the name of the sequence affected. One of our Product Specialists will be able to help troubleshoot further! :)