If you find a unit is showing grey areas on the Availability Chart it may be due to unconfigured rates. You can check this by navigating to Setup > Rates > Daily/Weekly/Short Breaks.
Select the affected unit, and check there is a date band covering the area which showed as grey on the chart. If there is no band you can create one by clicking on the ‘Add Rate’ button. Once the details have been entered click ‘Add’ then refresh the availability. It should now display as green on the chart.