There are many good thesis possibilities for The Giver. The book has so much to teach us about how not to live. Let's look at a few possibilities.
One potential thesis from The Giver is that not having choices is a form of slavery. The people in the story have few choices in life. What their careers are going to be is decided for them. Whom they will partner with to raise children with is not within their control. They are "given" children and have no choice about this. Even who will bear the children is decided by the Elders. This is, in effect, a community of slaves.
Another possibility is the thesis that people are not fully human if they are prevented from experiencing emotions. The people in The Giver have had all of their emotions repressed. They do not experience hatred, jealousy, or envy. That doesn't sound too terrible. On the other hand, though, they do not experience sexual longings. Nor do they experience love. There can be no true happiness without unhappiness, and the story supports this thesis very well.
Still another thesis is that sameness is a dreadful idea, taking away the wonderful variability of life. This is true in ways large and small. There are no hills and valleys in The Giver. There is no stormy weather. People's clothing, housing, and furnishings are all the same. There is no color. There is a schedule from which there are few deviations. This kind of sameness would be terrible for anyone to live with, since while we do feel best with some routine, we also need some variability.
Each of these theses can be fully supported from the text, and when you write your thesis statement, remember that it must include your thesis and your supporting points, too. For example, here is a thesis statement I might write for The Giver:
The Giver shows that depriving people of memory is a way of dehumanizing and endangering them.
That gives me two points to make in my essay, one on dehumanizing people and other on endangering them. I can support my idea now with examples of each from the story. For your thesis statement, you can do the same, with your thesis, your main idea, and your supporting points.