What is the main message of the novel The Giver?

The main message of the novel is that choice is not destructive.  In this society, the absence of choice is actually more destructive.  All choices are made for people, and as a result they act in inhume and immoral ways and don’t even know it.

One example of a commonly accepted behavior that we would consider immoral is the way family units are devised.  Children are created somehow (most likely they are genetically engineered and artificially implanted), but are taken from Birthmothers. 

"Anyway, Lily-billy," he said affectionately, "the Birthmothers never even get to see newchildren. If you enjoy the little ones so much, you should hope for an Assignment as Nurturer." (ch 3, p. 22)

The children are then assigned to couples based on applications, and couples are matched as well.  There is a complete absence of love.

Another problem is release.  If a person breaks three rules, the person is released.  If a baby is not big enough, it is released.  When people get too old, they are released.  When Jonas finds out that release means to kill someone, he is horrified.

"I will do whatever you like, sir. I will kill people, sir. Old people? Small newborn people? I'd be happy to kill them, sir. Thank you for your instructions, sir. (ch 20, pp. 152-153).

Everyone in the community is unquestionably obedient.  Things are the way they are, and always have been.  No one imagines things could ever be different.

There's nothing. There's nothing we can do. It's always been this way. Before me, before you, before the ones who came before you. Back and back and back." (ch 20, p. 154)

When Jonas receives the memories from The Giver, he begins to understand that there is real trouble in the community.  At this point, he becomes discouraged because he does not think things will change.  The Giver shows him that things can be different, and encourages Jonas to choose to leave so the memories can be returned to the people.

Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.