Please can you do the below first and remove each crank from the axle, make sure when fitting the crank back onto the axle you push the crank as hard onto the axle as you can and then really tighten the screws. 
In the assembly instructions, steps 14 & 15 show how to install the right crank arm onto the axle. If the crank arm is not pressed on tight enough in step 14 before tightening the screws in step 15 then there will be a once-per-revolution knock felt in the crank arm. Also, if the screws in step 15 are not completely tight that could also create a knocking sensation as you pedal.

Please remove the 3 screws from the right crank arm, remove the crank arm, and repeat steps 14 & 15. Once finished, confirm that both screws in step 15 are very tight. If the knocking persists, repeat the previous steps with the left side crank arm. #