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The property 'classList' is unable to be read for undefined in a Bootstrap 5.1 Modal popup because the element that the 'classList' property is being accessed on is not defined or does not exist. This can happen if the Modal is not initialized properly, or if the element that is supposed to trigger the Modal is not found in the DOM. Without a valid HTML element, there is no object for the 'classList' property to be accessed on, resulting in the error that the property is undefined.