"The poem "Mending Wall", by Robert Frost deals with human barriers." Do you agree?

Definitely. I think you need to be aware of the way that
Frost presents the ambiguity over the wall. The speaker is very open about his distaste
for walls, saying that he doesn't love them and he presents the wall in a way that seems
to emphasise the way that it divides people. Note the following
example:

And
on a day we meet to walk the line

And set the wall between
us once again.

We keep the wall between us as we
go.

Note how the wall is what
separates the speaker from his neighbour and prevents them from being together. Even
when the speaker feels that they do not need a wall as their territory is clearly marked
by the different trees, the neighbour presents his opposing
argument:

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"Good fences make good
neighbours."

However, this
leads the speaker to make a series of very thoughtful and pertinent questions and
statements, such as this:

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Before I built a wall I'd ask to
know

What I was walling in or walling
out,

And to whom I was like to give
offence.

This quote above all
cements the way in which this poem is about human divisions. Building any form of wall
means that you are implicitly walling in some people and walling out others, thereby
causing offence. Frost therefore symbolically writes about how we built walls, real or
metaphorical, that may give us a sense of security but also separate us and prevent
communication and true togetherness.