If you open your goal book, is the goal Honey Garden Part 4 still showing as completed, meaning is the goal no longer in your book?
If no - then your game had a sync error and doesn't realize you finished the final part of the goal and will only give you the prize if you complete whichever tasks are left unchecked. Since you likely won't have time to do this, you'll have to open a Support Ticket to contact the team directly to see if they can help you.
If yes - this will sound silly but make sure you're looking in the correct area of your inventory and be sure you're flipping through the pages slowly. The Chick Pen will not be in the back of the inventory with the other goal rewards because it was originally released as a normal Market Purchase item, it will be well into the middle of the list. On my iOS device it is between Dragon Baby and Flanta floor decos. If you go through your inventory carefully and are absolutely sure you've not been awarded the item, you'll have to open a Support Ticket to contact the team directly.