R-4.5.1 is compatible with both release and devel versions of Bioconductor. Which one are you using? I am still on release, and it's fine:

> BSgenome.Hsapiens.UCSC.hg38
| BSgenome object for Human
| - organism: Homo sapiens
| - provider: UCSC
| - genome: hg38
| - release date: 2023/01/31
| - 711 sequence(s):
|     chr1                    chr2                    chr3                   
|     chr4                    chr5                    chr6                   
|     chr7                    chr8                    chr9                   
|     chr10                   chr11                   chr12                  
|     chr13                   chr14                   chr15                  
|     ...                     ...                     ...                    
|     chr19_KV575256v1_alt    chr19_KV575257v1_alt    chr19_KV575258v1_alt   
|     chr19_KV575259v1_alt    chr19_KV575260v1_alt    chr19_MU273387v1_alt   
|     chr22_KN196485v1_alt    chr22_KN196486v1_alt    chr22_KQ458387v1_alt   
|     chr22_KQ458388v1_alt    chr22_KQ759761v1_alt    chrX_KV766199v1_alt    
|     chrX_MU273395v1_alt     chrX_MU273396v1_alt     chrX_MU273397v1_alt    
| 
| Tips: call 'seqnames()' on the object to get all the sequence names, call
| 'seqinfo()' to get the full sequence info, use the '$' or '[[' operator to
| access a given sequence, see '?BSgenome' for more information.
> sessionInfo()
R version 4.5.0 (2025-04-11)
Platform: x86_64-pc-linux-gnu
Running under: Ubuntu 24.04.2 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 
LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0

locale:
[1] C

time zone: Etc/UTC
tzcode source: system (glibc)

attached base packages:
[1] stats4    stats     graphics  grDevices utils     datasets  methods  
[8] base     

other attached packages:
 [1] BSgenome.Hsapiens.UCSC.hg38_1.4.5 BSgenome_1.76.0                  
 [3] rtracklayer_1.68.0                BiocIO_1.18.0                    
 [5] Biostrings_2.76.0                 XVector_0.48.0                   
 [7] GenomicRanges_1.60.0              GenomeInfoDb_1.44.3              
 [9] IRanges_2.42.0                    S4Vectors_0.46.0                 
[11] BiocGenerics_0.54.0               generics_0.1.4                   

loaded via a namespace (and not attached):
 [1] Matrix_1.7-3                jsonlite_2.0.0             
 [3] compiler_4.5.0              rjson_0.2.23               
 [5] crayon_1.5.3                SummarizedExperiment_1.38.1
 [7] Biobase_2.68.0              Rsamtools_2.24.1           
 [9] bitops_1.0-9                GenomicAlignments_1.44.0   
[11] parallel_4.5.0              BiocParallel_1.42.1        
[13] yaml_2.3.10                 lattice_0.22-7             
[15] R6_2.6.1                    S4Arrays_1.8.1             
[17] curl_6.2.2                  XML_3.99-0.19              
[19] DelayedArray_0.34.1         MatrixGenerics_1.20.0      
[21] GenomeInfoDbData_1.2.14     SparseArray_1.8.1          
[23] grid_4.5.0                  codetools_0.2-20           
[25] abind_1.4-8                 RCurl_1.98-1.17            
[27] restfulr_0.0.16             httr_1.4.7                 
[29] matrixStats_1.5.0           tools_4.5.0                
[31] UCSC.utils_1.4.0           
> library(BiocManager)
Bioconductor version 3.21 (BiocManager 1.30.25), R 4.5.0 (2025-04-11)

Thank you for your replies. I got the problem solved by installing mutSignature with this line:

 devtools::install_github("dami82/mutSignatures", force = TRUE, build_vignettes = FALSE)

Looks like there are discrepancies between the CRAN source and the GitHub source. Package installed from CRAN has the following line:

if ((! "BSgenome" %in% class(BSGenomeDb))  | 
      sum(c("pkgname", "organism") %in% methods::slotNames(BSGenomeDb)) != 2)
    stop ("Please, provide a valid 'BSgenome'-class object")

This is the cause for the problem since "organism" is not in the slotNames(BSGenomeDb) as shown by the output below:

> slotNames(BSgenome.Hsapiens.UCSC.hg38)
[1] "pkgname"            "single_sequences"   "multiple_sequences" "seqinfo"            "user_seqnames"      "injectSNPs_handler"
[7] ".seqs_cache"        ".link_counts"       "metadata"

Maybe slotNames() should not be used in checking the BSGenomeDb object in mutSignature. Any way, I' glad this issue is solved.