I get an error, not sure what's wrong there... Hope the authors of the package can make the error message more informative.

requireUniProt.ws)

up <- UniProt.ws(taxId=9606)

res <- select(up,
              keytype = "UNIPROTKB",
              keys = c('Q9NTX7', 'P19484', 'Q6ZSR9', 'A6NKZ8'),
              columns = c('ENTREZ_GENE', 'GENES', 'GENEID')
              )
# 
# Error in .resortColumns(tab, jointype, reqCols) : 
#   [internal] some of 'reqCols' are not in 'tab'
# In addition: Warning message:
#   In `[<-.factor`(`*tmp*`, iseq, value = c("Q9NTX7", "P19484", "Q6ZSR9",  :
#                                              invalid factor level, NA generated

UPD. adding sessionInfo():

R version 3.6.1 (2019-07-05)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18363)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] UniProt.ws_2.26.0   BiocGenerics_0.32.0 RCurl_1.98-1.2      RSQLite_2.2.0      

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.4.6         rstudioapi_0.11      AnnotationDbi_1.48.0 magrittr_1.5         rappdirs_0.3.1      
 [6] IRanges_2.20.2       tidyselect_1.1.0     bit_4.0.4            R6_2.4.1             rlang_0.4.7         
[11] httr_1.4.2           dplyr_1.0.1          blob_1.2.1           tools_3.6.1          Biobase_2.46.0      
[16] DBI_1.1.0            dbplyr_1.4.4         ellipsis_0.3.0       assertthat_0.2.1     bit64_4.0.2         
[21] digest_0.6.25        tibble_3.0.1         lifecycle_0.2.0      crayon_1.3.4         purrr_0.3.4         
[26] vctrs_0.3.2          S4Vectors_0.24.4     bitops_1.0-6         curl_4.3             glue_1.4.0          
[31] memoise_1.1.0        BiocFileCache_1.10.2 compiler_3.6.1       pillar_1.4.6         generics_0.0.2      
[36] stats4_3.6.1         pkgconfig_2.0.3

It looks like you are using an older version of UniProt.ws from Bioc3.10 and I was able to reproduce your error. However, on the current release version UniProt.ws (2.28.0) this error is not present, which most likely means that the issue was fixed sometime between your version and the current one. To fix this I would update your R version to 4.0, update your Bioconductor version to 3.11, and use UniProt.ws version 2.28.0.

> up <- UniProt.ws(taxId=9606)
> res <- select(up, 
    keytype = "UNIPROTKB", 
    keys = c('Q9NTX7', 'P19484', 'Q6ZSR9', 'A6NKZ8'), 
    columns = c('ENTREZ_GENE', 'GENES', 'GENEID'))
GENEID and ENTREZ_GENE are the same.
  returning only GENEID in results.
Getting mapping data for Q9NTX7 ... and P_ENTREZGENEID
Getting extra data for Q9NTX7, P19484, Q6ZSR9
'select()' returned 1:1 mapping between keys and columns
> res
  UNIPROTKB        GENES GENEID
1    Q9NTX7       RNF146  81847
2    P19484 TFEB BHLHE35   7942
3    Q6ZSR9         <NA>   <NA>
4    A6NKZ8         <NA>   <NA>
>
> BiocManager::version()
[1] ‘3.11’
> sessionInfo()
R version 4.0.0 alpha (2020-04-07 r78171)
Platform: x86_64-apple-darwin17.7.0 (64-bit)
Running under: macOS High Sierra 10.13.6

Matrix products: default
BLAS:   /Users/ka36530_ca/R-stuff/bin/R-4-0/lib/libRblas.dylib
LAPACK: /Users/ka36530_ca/R-stuff/bin/R-4-0/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods
[8] base

other attached packages:
[1] UniProt.ws_2.28.0   BiocGenerics_0.34.0 RCurl_1.98-1.2
[4] RSQLite_2.2.0

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.5           pillar_1.4.6         compiler_4.0.0
 [4] BiocManager_1.30.10  dbplyr_1.4.4         bitops_1.0-6
 [7] tools_4.0.0          digest_0.6.25        bit_4.0.4
[10] BiocFileCache_1.12.1 memoise_1.1.0        lifecycle_0.2.0
[13] tibble_3.0.3         pkgconfig_2.0.3      rlang_0.4.7
[16] DBI_1.1.0            curl_4.3             dplyr_1.0.2
[19] httr_1.4.2           generics_0.0.2       S4Vectors_0.26.1
[22] vctrs_0.3.4          IRanges_2.22.2       rappdirs_0.3.1
[25] stats4_4.0.0         bit64_4.0.5          tidyselect_1.1.0
[28] glue_1.4.2           Biobase_2.48.0       R6_2.4.1
[31] AnnotationDbi_1.50.3 purrr_0.3.4          blob_1.2.1
[34] magrittr_1.5         ellipsis_0.3.1       assertthat_0.2.1
[37] crayon_1.3.4
>