If you can, avoid getting stuck by reading this post:
https://steamproxy-script.pipiskins.com/app/665510/discussions/0/3247562523075443125/

But if you still forget about it and hit the bug (like I did :P), then there's still a chance to fix it, IF:

1) you don't want to start over your game
2) you know how to use an Hex-editor

If you can't use an hex-editor then it might be faster to actually restart the game with a new profile, since I'm not going to explain anything about it.

1) Find your savegame.
Browse to: %APPDATA%\Urchin\Vengeance\VengeanceBFG\Profiles
The savegame is the biggest file (in my system it's 12kb, and with an .upf extension)

2) Open it with your hex-editor and find the following string (ASCII): BoxTile1 (if you can't find it then it might be the wrong file)

3) You'll notice a number in the ASCII column after the string and the 0D0A sequence, it should be a 1.
Change it to 4

4) Save the file and exit the editor, and when you'll load your game you should have the box tile in your inventory.