“If I want to create a plane using Unreal Engine that is completely transparent but can block a specific object of my choice, how can this be achieved?”
You can create a blocking volume: Volumes Reference | Unreal Engine 4.27 Documentation. This should be enough for your purposes.
If you insist on having a plane, you could create a material that is completely transparent. The block and collision will still be in place making sure the character can’t pass through.