You have an email with at least one attachment and choose to print only the body of the email.
Outlook tries to print the attachment and either gives an error or prompts for an action.
'Print Attached Files' is selected in the Outlook print settings.
Follow the steps below:
File-Print and choose Native Print
Un-tick this box and click ‘Preview’
You can also make this change in the registry but Outlook must be restarted to take effect in this case.
Set ‘Print Attachments’ = 0
This could be set via Group Policy.