When they get this message it means that they are trying to apply to a board that is set to not accept applications.
In the Application Settings you can choose from the following 3 options that will apply for all boards:
Always - you accept applications anytime regardless of if there are any seat vacancies or not.
Only when vacancies exist - The applicant can apply for a board if there are any empty seats.
X Days before Vacancy - The applicant can apply for a board if there is a empty seat or a seat that has a term expiring within X amount of days (You can set the number of days in the field below this).
In addition you can set applications by board.
No - The board will not accept applications
Yes - The board will allow applications (depending on the global settings)
Per Seat - This allows you to accept applications for specific seats on a board (depending on the global settings). For example if a board requires a qualified specialist like a pilot for an airport advisory board you can set the application form to show that you are only accepting a certain person for that seat.