The metal rings are not tighten right. Here are some simple steps and tips to fix this issue:
1: Tight the metal ring more harder.
2: The attached PTFE tape is the best product for stopping the leaking of threads. (know more)
3: switch the rings or canisters, sometime one thread could match more better with another one.
Video link on youtube: https://www.youtube.com/watch?v=zTXxxnd3bp8
The RO System manual :