Hey there! I'm happy to help!
The prefix ob- has three main meanings: thoroughly, toward, or in the way. If something is obstructive, it covers up something or gets in the way. This means that the best meaning of this prefix in this case is in the way. Struct usually means to build. So, if something is obstructive, it is literally in the way of building or it hinders it.
Have a wonderful day! :D
