Luke gave some people street directions, but because they only nodded their heads and didn't repeat the directions back to him, he doesn't really know whether the directions were understood. What type of barrier is present here?

Answer: feedback barrier to effective communication.

Explanation:

When a person receives message they give feedback based on what they interpreted and understood about the message that they received. Their feedback may create barrier based on several factors: the fear of the person giving the message, generalised feedback which doesn't respond specifically to the message received, feedback that doesn't give the sender enough information to know if further explanation of the message is needed.

Some people may be afraid to say they didn't understand what was communicated with the fear that maybe most people understood and they may look foolish if they say they didn't understand.

It may also happen that these people didn't understand Luke's language and didn't know how to say that back but just pretended as if they understood.