Error when trying to change the selection mode from "Single" to "Multiple"

Summary: I opened a Sandbox selecting page composer, and when I tried to change the selection mode of the "Copy Other Recipients" field from "Single" to "Multiple". I would like to copy more than one recipient on the performance evaluation overdue task email, In addition to the task owner. To to that, I tried to change that field using the selection mode, but I am not sure if the value "Multiple" is correct.

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):