My question is similar to Cmdstanr under R4.3 and Rtools 4.3 - Interfaces / CmdStan - The Stan Forums (mc-stan.org) but I am a Linux-based, cloud hosted (by my employer) Posit RStudio Pro.
Basically, cmdstanr works under 4.2.2, but will not compile not 4.3.0. However, if I compile the file under 4.2.2, it will run under 4.3.0
I think I am using the newest version of cmdstanr and Stan  from GitHub.
Here is my error on 4.3:
> occ_3_hm <- cmdstan_model("../occstan_hm/stan_files/occupancy_hm_3_a.stan",
+                           cpp_options = list(stan_threads = TRUE))
Compiling Stan program...
Syntax error in '/tmp/RtmprLVa83/model-14661544256.stan', line 2, column 0, include error:
   -------------------------------------------------
     1:  functions{
     2:  #include /scale_cholesky_factor.stan
         ^
     3:  #include /occupancy_3_function.stan
     4:  #include /detect_3.stan
   -------------------------------------------------
Could not find include file '/scale_cholesky_factor.stan' in specified include paths.
Current include paths: None
make: *** [make/program:50: /tmp/RtmprLVa83/model-14661544256.hpp] Error 1
Error: An error occured during compilation! See the message above for more information.
Session info not working:
> sessionInfo()
R version 4.3.0 (2023-04-21)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.6 LTS
Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0 
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0
locale:
 [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8       
 [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8   
 [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C          
[10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   
time zone: Etc/UTC
tzcode source: system (glibc)
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     
other attached packages:
 [1] cmdstanr_0.5.3  ggthemes_4.2.4  lubridate_1.9.2 forcats_1.0.0   stringr_1.5.0  
 [6] dplyr_1.1.2     purrr_1.0.1     readr_2.1.4     tidyr_1.3.0     tibble_3.2.1   
[11] ggplot2_3.4.2   tidyverse_2.0.0
loaded via a namespace (and not attached):
 [1] tensorA_0.36.2       utf8_1.2.3           generics_0.1.3       stringi_1.7.12      
 [5] hms_1.1.3            magrittr_2.0.3       grid_4.3.0           timechange_0.2.0    
 [9] jsonlite_1.8.7       processx_3.8.2       backports_1.4.1      ps_1.7.5            
[13] fansi_1.0.4          scales_1.2.1         abind_1.4-5          cli_3.6.1           
[17] rlang_1.1.1          crayon_1.5.2         bit64_4.0.5          munsell_0.5.0       
[21] withr_2.5.0          parallel_4.3.0       tools_4.3.0          tzdb_0.4.0          
[25] checkmate_2.2.0      colorspace_2.1-0     vctrs_0.6.3          posterior_1.4.1     
[29] R6_2.5.1             lifecycle_1.0.3      bit_4.0.5            vroom_1.6.3         
[33] pkgconfig_2.0.3      pillar_1.9.0         gtable_0.3.3         glue_1.6.2          
[37] xfun_0.39            tidyselect_1.2.0     rstudioapi_0.14      knitr_1.43          
[41] farver_2.1.1         bayesplot_1.10.0     compiler_4.3.0       distributional_0.3.2
This same code works with R 4.2.2, session info working:
> sessionInfo()
R version 4.2.2 (2022-10-31)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.6 LTS
Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0
locale:
 [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8       
 [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8   
 [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C          
[10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     
other attached packages:
 [1] cmdstanr_0.5.3  ggthemes_4.2.4  forcats_1.0.0   stringr_1.5.0   dplyr_1.1.0    
 [6] purrr_1.0.1     readr_2.1.4     tidyr_1.3.0     tibble_3.2.1    ggplot2_3.4.2  
[11] tidyverse_1.3.2
loaded via a namespace (and not attached):
 [1] lubridate_1.9.2      ps_1.7.5             assertthat_0.2.1     utf8_1.2.3          
 [5] R6_2.5.1             cellranger_1.1.0     backports_1.4.1      reprex_2.0.2        
 [9] httr_1.4.4           pillar_1.9.0         rlang_1.1.0          googlesheets4_1.0.1 
[13] readxl_1.4.2         data.table_1.14.8    rstudioapi_0.14      checkmate_2.1.0     
[17] googledrive_2.0.0    bit_4.0.5            munsell_0.5.0        broom_1.0.3         
[21] compiler_4.2.2       modelr_0.1.10        xfun_0.38            pkgconfig_2.0.3     
[25] tidyselect_1.2.0     tensorA_0.36.2       fansi_1.0.4          crayon_1.5.2        
[29] tzdb_0.3.0           dbplyr_2.3.0         withr_2.5.0          grid_4.2.2          
[33] distributional_0.3.2 jsonlite_1.8.4       gtable_0.3.3         lifecycle_1.0.3     
[37] DBI_1.1.3            magrittr_2.0.3       posterior_1.4.1      scales_1.2.1        
[41] cli_3.6.1            stringi_1.7.12       vroom_1.6.1          farver_2.1.1        
[45] fs_1.6.1             xml2_1.3.3           ellipsis_0.3.2       generics_0.1.3      
[49] vctrs_0.6.2          tools_4.2.2          bit64_4.0.5          glue_1.6.2          
[53] hms_1.1.2            parallel_4.2.2       processx_3.8.1       abind_1.4-5         
[57] timechange_0.2.0     colorspace_2.1-0     gargle_1.3.0         bayesplot_1.10.0    
[61] rvest_1.0.3          knitr_1.42           haven_2.5.1    
I guess my question is, what should I do? I’ve tried (last week):
- Upgrading 
cmdstanr - Reinstalling/upgrading Stan
 
So, right now I’m just using the old version of R.