It does appear that AJUBA is not represented on the Affymetrix Human Genome U133A Array. Furthermore, the code example you provide is incorrect. if you inspect xx you'll notice the names of the list elements are the probe IDs, which will always return NA/NULL. Here is the code I used to determine if AJUBA was represented on the array. Lastly, it looks like AJUBA is on the U133 plus 2 array.

library(org.Hs.eg.db)
library(hgu133a.db)
library(hgu133plus2.db)

x <- hgu133aENSEMBL
mapped_genes <- mappedkeys(x)
xx <- as.list(x[mapped_genes])
# $`1053_at`
# [1] "ENSG00000049541"
# 
# $`117_at`
# [1] "ENSG00000173110"
# 
# $`121_at`
# [1] "ENSG00000125618"
# 
# $`1255_g_at`
# [1] "ENSG00000048545"
# 
# $`1316_at`
# [1] "ENSG00000126351"

#First get common identifiers for AJUBA
ajuba <- AnnotationDbi::select(org.Hs.eg.db, "AJUBA", columns=c("REFSEQ","ENTREZID","SYMBOL","ENSEMBL","ALIAS"), keytype = "SYMBOL")

head(ajuba)
# SYMBOL       REFSEQ ENTREZID         ENSEMBL ALIAS
# 1  AJUBA NM_001289097    84962 ENSG00000129474   JUB
# 2  AJUBA NM_001289097    84962 ENSG00000129474 AJUBA
# 3  AJUBA    NM_032876    84962 ENSG00000129474   JUB
# 4  AJUBA    NM_032876    84962 ENSG00000129474 AJUBA
# 5  AJUBA    NM_198086    84962 ENSG00000129474   JUB
# 6  AJUBA    NM_198086    84962 ENSG00000129474 AJUBA

#Loop over each set of identifiers and query the array annotation db.
#I had to warp the select in a tryCatch because the ENTREZID was throwing an error because the it is not in the array
rst <- lapply(colnames(ajuba), function(x){
  tryCatch(select(hgu133a.db, unique(ajuba[, x]), columns = c("PROBEID"), keytype = x), 
           error=function(e) return("error")) 
})
sapply(rst, NROW) #check oh many rows are returned from querying each column (not promising)
# [1] 0 0 1 0 0

#To make sure I didn't miss something, I also search the Affy Human U133 plus 2 array.
rst2 <- lapply(colnames(ajuba), function(x){
  tryCatch(select(hgu133plus2.db, unique(ajuba[, x]), columns = c("PROBEID"), keytype = x), 
           error=function(e) return("error")) ##I had to warp the select in a tryCatch because the ENTREZID was throwing an error 
})

sapply(rst2, NROW) #Looks like there are probes that map to AJUBA.
# [1]  4 24  4  4  8

# lapply(rst2, head)
# [[1]]
# SYMBOL      PROBEID
# 1  AJUBA 1553764_a_at
# 2  AJUBA    225806_at
# 3  AJUBA    225807_at
# 4  AJUBA    243446_at
# 
# [[2]]
# REFSEQ      PROBEID
# 1 NM_001289097 1553764_a_at
# 2 NM_001289097    225806_at
# 3 NM_001289097    225807_at
# 4 NM_001289097    243446_at
# 5    NM_032876 1553764_a_at
# 6    NM_032876    225806_at
# 
# [[3]]
# ENTREZID      PROBEID
# 1    84962 1553764_a_at
# 2    84962    225806_at
# 3    84962    225807_at
# 4    84962    243446_at
# 
# [[4]]
# ENSEMBL      PROBEID
# 1 ENSG00000129474 1553764_a_at
# 2 ENSG00000129474    225806_at
# 3 ENSG00000129474    225807_at
# 4 ENSG00000129474    243446_at
# 
# [[5]]
# ALIAS      PROBEID
# 1   JUB 1553764_a_at
# 2   JUB    225806_at
# 3   JUB    225807_at
# 4   JUB    243446_at
# 5 AJUBA 1553764_a_at
# 6 AJUBA    225806_at