Why was Jonas not given a life assignment at the the age of twelve in the The Giver?

You are right to identify that in Chapter Seven, Jonas is
left alone in his group of peers as the only one who has not received an assignment. If
you look at the end of the chapter, this is something that clearly makes him very
uncomfortable and also causes him to consider what he has done
wrong:

He
hunched his shoulders and tried to make himself smaller in the seat. He wanted to
disappear, to fade away, not to exist. He didn't dare to turn and find his parents in
the crowd. He couldn't bear to see their faces darkened with
shame.

However, as Chapter
Eight begins, we see that this feeling of shame and isolation ends and is replaced by
something infinitely better, as we discover that Jonas has been singled out for a unique
job that only one other person in the community has: the Receiver of the community. The
Receiver of Memory is not assigned, as the Chief Elder says, but "selected." The
difference between this job and others is explained by the Chief
Elder:

"But
the Receiver-in-training cannot be observed, cannot be modified. This is stated quite
clearly in the rules. He is to be alone, apart, while he is prepared by the current
Receiver for the job which is the most honoured of our
community."

So, Jonas has not
been assigned because he has been selected to perform the most important job of the
entire community. As he soon discovers, the normal rules do not apply to him, and he is
given considerable leniency.