Why can I not click the drop frame tab in composition settings?

You have a misunderstanding about the timecode. Drop Frame timecode is only available when the comp framerate is 29.97 or 59.94 frames per second. Drop Frame timecode does not drop frames, it drops frame numbers so the timecode display is accurate to the time. You never want to use Non-Drop Frame timecode because the displayed time will be off as soon as the comp is longer than one minute. Drop Frame timecode drops frame numbers 0 and 1 on the first frame of every minute except when the number of minutes is divisible by 10. It was invented when they had to add extra scan lines to television to handle color broadcasts. It was the only way to keep the time correctly to the nearest frame. 

Change the comp frame rate to 29.97 or 59.94 and you will have access to the option. Make sure that you use Drop Frame timecode so the time display of your comp is accurate.

This only applies to fractional framerates and comes down to the old NTSC math and how timecodes are calculated. It does not actually "drop" any frames. If you want that, you can use Posterize Time, Time remapping and other effects like Time-Displacement.

Mylenium