I keep on getting this warning message when I try to install cytofkit in R version 3.6.3.

    > BiocManager::install("cytofkit")
Bioconductor version 3.9 (BiocManager 1.30.10), R 3.6.3 (2020-02-29)
Installing package(s) 'cytofkit'
Installation path not writeable, unable to update packages: foreign, lattice, nlme, nnet
Old packages: 'fs', 'plyr'
Update all/some/none? [a/s/n]: 
a

  There are binary versions available but the source versions are later:
     binary source needs_compilation
fs    1.3.1  1.3.2              TRUE
plyr  1.8.5  1.8.6              TRUE

  Binaries will be installed
trying URL 'https://cran.rstudio.com/bin/windows/contrib/3.6/fs_1.3.1.zip'
Content type 'application/zip' length 982859 bytes (959 KB)
downloaded 959 KB

trying URL 'https://cran.rstudio.com/bin/windows/contrib/3.6/plyr_1.8.5.zip'
Content type 'application/zip' length 1304574 bytes (1.2 MB)
downloaded 1.2 MB

package ‘fs’ successfully unpacked and MD5 sums checked
package ‘plyr’ successfully unpacked and MD5 sums checked

The downloaded binary packages are in
    C:\Users\AppData\Local\Temp\RtmpUBAsIj\downloaded_packages
Warning message:
package ‘cytofkit’ is not available (for R version 3.6.3)

I have tried to use R version 3.5, but when I do this I get an error for caTools, saying that it requires a version of R (≥ 3.6.0). Is there any solution to this problem? I am new to R, so I am not aware of other methods. Thank you in advance.

cytofkit was removed from Bioconductor in version 3.9 http://bioconductor.org/about/removed-packages/ so yes you must use R 3.5.

cytofkit was removed for a reason (because it failed to pass build & check) so it is at some level known to be 'broken' and therefore not a good choice. If you still want to use it, you could try installing an earlier version of caTools from https://cran.r-project.org/src/contrib/Archive/caTools/ ; because the package has C code, you would need to install the Windows compiler tool chain https://cran.r-project.org/bin/windows/Rtools/, download the archived version, then install from 'source' with install.packages("caTools_1.17.1.tar.gz") (I'm not certain of the version number), and then install cytofkit using BiocManager::install(). This would definitely be a hard-core but not impossible exercise.