Employees can only receive one invitation email daily to avoid spamming. Hence, you will only be able to resend the invitation once daily. 

The resending invitation attempt fails when you have previously tried to reinvite that employee on the same day. You can try the same action the next day. 

If you feel that you haven't reinvite any employee, but the resending invitation attempt you did fail, please check with your fellow admins. Your fellow admins may have reinvited that particular employee. You can try the same action the next day.