This is most likely because the vanilla Minecraft launcher has not installed correctly. The easiest way to solve this is to remove the current installation of it and then reinstall it. You can do this by first finding the directory that it is installed in, to do this first click the "Open folder" button on the pack details page.

This will then open Windows Explorer set to the instance that you are having problems with. Once you are here you can browse to the .ftba folder by clicking on .ftba in the address bar.

With the just the bin folder selected, press the delete key to delete it. Now with the bin directory deleted you should proceed to reinstall the pack How do I install a Modpack?, as part of the pack installation process the bin folder will be recreated.