When the "Use default Email" checkbox is selected, the "To" and "CC" fields will use the values set by "Set default Email". The user must set values for "To" and "CC" fields in the "Set Default Email" option before selecting the "Use default Email" checkbox. If this has not been done previously, then the "To" and "CC" fields will be empty and it will not allow the panel to be saved when the "Use default Email" checkbox is selected.
After the "To" and "CC" fields" is set in the "Set default Email" dialog under the "All Alarms" alarm setting, like below:

Then when the checkbox "Use default Email" is selected, the dialog "Message Settings" can be saved successfully.