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.
I am not certain that I can find ONE main message in this complex book. The Giver includes the idea of blind obediance to a group (as in the people expecting Jonas to carry all of the memories for the group) which can be viewed (the forcing of those who do not comply or are flawed into release - an inferred death) as dangerous, as well as sacrifice for a group. I feel that the idea of self sacrifice for another becomes exceptionally important when Jonas leaves the community to save baby Gabriel. It is also his "job" in the community. Jonas must suffer for the group so that they do not have to. He is the one who receives all of the painful memories and hurts, not the community. A final message might be the question "Can a true utopia exhist?" Jonas lives in one, but is it the best thing for everyone if they can never feel true feelings, never have true happiness or sadness? Never see those who are different from them, or have a difference of opinion?
It might be interesting for you to read A Gathering Blue which MAY be considered Lowry's sequel.