Why is the rerack task "greyed out"?

In order to undertake a rerack task, you need to have at least two casks on the picking list.  This task type requires more than one cask, hence why it was perhaps showing as greyed out if you only had one cask on your picking list.

You can do a 1-to-1 rerack, many-to-1 or 1-to-many rerack.  

Once you have the correct number of casks ready to perform your re-rack in your picking list, it should be clear when creating the task, the selected options will be presented for the reracking operation you're looking to undertake.  

If doing these in larger quantities, it may be preferrable to undertake a disgorge and filling task.