The reason you can't pick up the medikit is explained both in the game description and within the game itself. In order to use the medikit, you must be in the countdown phase, which occurs when your rage level reaches its maximum.
Additionally, the limitation of only being able to attack with the left mouse button is also detailed in the game and its description. If you wish to attack while moving, there's a dash feature mentioned in the controls section of the description. This dash can be activated whether you are standing still or in motion.
Furthermore, there is a known bug related to the death cinematics caused by the encoding of the video. Unfortunately, when you lose, you will need to close and reopen the game to continue. I have already resolved this bug; however, I am unable to update the game due to restrictions on updates.
I appreciate your understanding, and I wish you all the best!