Can't get the key to open the chest even after doing all objectives

after diong all objectives,claptrap gives u a small speach,go to were your reward awaits for u,did u get all collectables as well^^,when u have u can walk over to the gold chest and press your use key

Finally found all the collectibles. I've been playing the map for a long while now, hoping that the last collectible would be in front of me in a place I never thought of. Of course, that was the case.
I created a video of the collectible locations for anyone interested.
https://www.youtube.com/watch?v=7Jd7ezldozI