Open oil drain solenoid and clean solenoid plunger with dry soft clots and replace it. After that check the apply vacuum, if you still have same issue then you may need to replace oil drain solenoid.
Hope this will help to solve your problems.
Thank.
may be the cause is no power source, try to check voltage reading at your breaker, if the breaker itself has a power then the problem cause at your ROBINAIR, if you have guts to open the unit ?, try it and find which cause the trouble , thank you and GOD BLESS.