Hello Phakin Bovorndamrongsin,
Happy to help!
One easy solution which would allow you to control when your learners will be able to seek through a video is to add a transparent object on top of a video that will only go away once they've completed the course. I've attached a sample Storyline 3 file so you can see how this can be done using trigger conditions and variables. Basically, the transparent object in the project (Right click -> Format Shape to make an object transparent) will change its state to 'Hidden' when the value of a variable in the second slide switches from False to True. The absence of the transparent object will now allow learners to seek through a video uninterrupted.
Since this is a design inquiry, I'll open the floor up to other members of the community so they can share how they addressed similar requirements.
Let me know if you have any questions!
