This error means that a validation has failed. A validation is a rule defined as part of an API Creator project. This rule is checked any time the relevant data is changed. If the rule is violated, then the request fails with this error code.
If you have admin access to this API Creator project, and you did not expect this behavior, you should probably check the rule definition and see why it evaluated to false.
If you do not have admin access to this project, you need to contact someone who understands the logic in this project, who can explain why this validation is in place, and what to do to avoid this error.