I'm very new to R and I'm running into the same problem over and over again. I tried already all solution suggestions I could find but nothing is working.

I try to install the BSgenome.Hsapiens:UCSC.hg19 using

BiocManager::install("BSgenome.Hsapiens.UCSC.hg19")

and I'll always get the same error warning.

BiocManager::install("BSgenome.Hsapiens.UCSC.hg19")
Bioconductor version 3.10 (BiocManager 1.30.10), R 3.6.1 (2019-07-05)
Installing package(s) 'BSgenome.Hsapiens.UCSC.hg19'
installing the source package ‘BSgenome.Hsapiens.UCSC.hg19’

versuche URL 'https://bioconductor.org/packages/3.10/data/annotation/src/contrib/BSgenome.Hsapiens.UCSC.hg19_1.4.0.tar.gz'
Content type 'application/x-gzip' length 688190187 bytes (656.3 MB)
downloaded 656.3 MB

"\\"
CMD.EXE wurde mit dem oben angegebenen Pfad als aktuellem Verzeichnis gestartet.
UNC-Pfade werden nicht untersttzt.
Stattdessen wird das Windows-Verzeichnis als aktuelles Verzeichnis gesetzt.
* installing *source* package 'BSgenome.Hsapiens.UCSC.hg19' ...
** using staged installation
** R
** inst
** byte-compile and prepare package for lazy loading
Fehler in findpack(package, lib.loc) : 
  there is no package called 'BSgenome.Hsapiens.UCSC.hg19'
Ruft auf: <Anonymous> -> findpack
Ausführung angehalten
ERROR: lazy loading failed for package 'BSgenome.Hsapiens.UCSC.hg19'
* removing '\\'

The downloaded source packages are in
    ‘C:\Users\montag\AppData\Local\Temp\Rtmpo3Vm0w\downloaded_packages’
Installation path not writeable, unable to update packages: boot, foreign, KernSmooth, Matrix, mgcv, nlme, survival
Old packages: 'cli', 'plyr'
Update all/some/none? [a/s/n]: 
a

  There are binary versions available but the source versions are later:
     binary source needs_compilation
cli   1.1.0  2.0.0             FALSE
plyr  1.8.4  1.8.5              TRUE

  Binaries will be installed
versuche URL 'https://cran.rstudio.com/bin/windows/contrib/3.6/plyr_1.8.4.zip'
Content type 'application/zip' length 1302229 bytes (1.2 MB)
downloaded 1.2 MB

package ‘plyr’ successfully unpacked and MD5 sums checked

The downloaded binary packages are in
    C:\Users\montag\AppData\Local\Temp\Rtmpo3Vm0w\downloaded_packages
installing the source package ‘cli’

versuche URL 'https://cran.rstudio.com/src/contrib/cli_2.0.0.tar.gz'
Content type 'application/x-gzip' length 111873 bytes (109 KB)
downloaded 109 KB

"\\"
CMD.EXE wurde mit dem oben angegebenen Pfad als aktuellem Verzeichnis gestartet.
UNC-Pfade werden nicht untersttzt.
Stattdessen wird das Windows-Verzeichnis als aktuelles Verzeichnis gesetzt.
* installing *source* package 'cli' ...
** Paket 'cli' erfolgreich entpackt und MD5 Summen überprüft
ERROR: cannot remove earlier installation, is it in use?
* removing '\\'
* restoring previous '\\'

The downloaded source packages are in
    ‘C:\Users\montag\AppData\Local\Temp\Rtmpo3Vm0w\downloaded_packages’
Warnmeldungen:
1: In install.packages(...) :
  installation of package ‘BSgenome.Hsapiens.UCSC.hg19’ had non-zero exit status
2: In install.packages(update[instlib == l, "Package"], l, repos = repos,  :
  installation of package ‘cli’ had non-zero exit status

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=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252
[4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252    

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

other attached packages:
[1] BSgenome_1.54.0      rtracklayer_1.46.0   Biostrings_2.54.0    XVector_0.26.0       GenomicRanges_1.38.0
[6] GenomeInfoDb_1.22.0  IRanges_2.20.1       S4Vectors_0.24.1     BiocGenerics_0.32.0 

loaded via a namespace (and not attached):
 [1] zlibbioc_1.32.0             GenomicAlignments_1.22.1    BiocParallel_1.20.0         lattice_0.20-38            
 [5] tools_3.6.1                 SummarizedExperiment_1.16.0 grid_3.6.1                  Biobase_2.46.0             
 [9] matrixStats_0.55.0          Matrix_1.2-17               GenomeInfoDbData_1.2.2      BiocManager_1.30.10        
[13] bitops_1.0-6                RCurl_1.95-4.12             DelayedArray_0.12.0         compiler_3.6.1             
[17] Rsamtools_2.2.1             XML_3.98-1.20

Please help me, I'm desperate at the moment

R doesn't like having the library directory on a network drive, so you have to trick it. Google 'windows map network drive' and follow the instructions that come up. You may need to adjust your .libPaths to point to the newly mapped drive.