Based on this information, what is Kiera's phenotype for the sickle-cell disease trait?
having sickle-cell disease not having sickle-cell disease
This passage describes the sickle-cell disease trait in humans:

In a group of humans, some individuals have sickle-cell disease and others do not. In this group, the gene for the sickle-cell disease trait has two alleles. The allele for not having sickle-cell disease (A) is dominant over the allele for having sickle-cell disease (a).
Kiera is a human from this group. Kiera has the homozygous genotype AA for the sickle-cell disease gene.
Kiera's genotype for the sickle-cell disease gene is AA. Kiera's genotype of AA has only A allelles. The A allele is for not having sickle-cell disease. So, Kiera's phenotype for the sickle-cell disease trait must be not having sickle-cell disease.
To check this answer, consider whether Kiera's alleles are dominant or recessive. The allele for not having sickle-cell disease (A) is dominant over the allele for having sickle-cell disease (a). This means A is a dominant allele, and a is a recessive allele.
Kiera's genotype of AA has two dominant alleles. An organism with at least one dominant allele for a gene will have the dominant allele's version of the trait. So, Kiera's phenotype for the sickle-cell disease trait must be not having sickle-cell disease.