"A wall has cracks"
Translation:Ukuta una nyufa
It's because of the noun class of ukuta. It's class 11/10 (U/N) so the verbal prefix is u-. The prefix a- is only used for people and animals.
M/WA (1/2) and other animate nouns regardless of class:
ana = s/he has
wana = they (people/animals) have
mti una = a tree has
miti ina = trees have
gari lina = a car has
magari yana = cars have
kitabu kina = a book has
vitabu vina = books have
nyumba ina = a house has
nyumba zina = houses have
ukuta una = a wall has
kuta zina = walls have
uzuri una = beauty has
kula kuna = eating has
hapa pana = this place has / there is ... here
huku kuna = this general area has / there is ... around here
humu mna = this internal area has / there is ... in here
Those subject prefixes are for all verbs.
By the way "to have" is not "kuna", it's "kuwa na" (literally "to be with") - it's irregular in the present tense and just uses the subject prefix directly on "with", but in any other tense it's regular: nina "I have"; nilikuwa na "I had".