I am trying to reserve a coral but the reserve button is grayed out and not clickable, why?

You must be signed into ReefTrader.com and you must click the "Attend" button for the specific swap that the coral is listed for.  In the black menu bar at the top of the screen, select the “Swaps” pull down menu, from there take a look at the swaps and select one.  You can also use the “Organizations” button from the top menu to search for an organization/club.  Selecting an organization will take you to their ReefTrader page with a list of their swaps where you can select a swap.    If you plan to attend the swap, click the “Attend” button.