The clue is the line saying "there is no package called 'dplyr'". DatastreamDSWS2R package has dependency on dplyr package, which you don't appear to have installed.
Install dplyr package from CRAN and then try installing DatastreamDSWS2R again.

Hello @Alex Putkov

Thank you! Installation seems to work now, but there is still a problem: I still cannot use basic formulas presented in the Getting started with R documentation, for example:

mydsws <-dsws$new()

I get the message:

Error: object 'dsws' not found

After install_github I get this output:

> install_github("CharlesCara/DatastreamDSWS2R", force = TRUE)
Downloading GitHub repo CharlesCara/DatastreamDSWS2R@master
These packages have more recent versions available.
Which would you like to update?

1: All                            
2: CRAN packages only             
3: None                           
4: zoo     (1.8-5 -> 1.8-6) [CRAN]
5: curl    (3.3   -> 4.0  ) [CRAN]
6: mime    (0.6   -> 0.7  ) [CRAN]
7: openssl (1.3   -> 1.4.1) [CRAN]
8: sys     (3.2   -> 3.3  ) [CRAN]

Enter one or more numbers, or an empty line to skip updates:
3
√  checking for file 'C:\Users\Maximilian\AppData\Local\Temp\Rtmp2z1Bho\remotes20f030ee247\CharlesCara-DatastreamDSWS2R-43480a9/DESCRIPTION'
-  preparing 'DatastreamDSWS2R':
√  checking DESCRIPTION meta-information ... 
-  checking for LF line-endings in source and make files and shell scripts
-  checking for empty or unneeded directories
-  looking to see if a 'data/datalist' file should be added
-  building 'DatastreamDSWS2R_1.5.12.tar.gz'
   
Installing package into ‘F:/Maximilian/Documents/R/win-library/3.4’
(as ‘lib’ is unspecified)
* installing *source* package 'DatastreamDSWS2R' ...
** R
** data
*** moving datasets to lazyload DB
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
*** arch - i386
*** arch - x64
* DONE (DatastreamDSWS2R)
> 

You need to load the package before you can call its members.

library(DatastreamDSWS2R)

It appears you've never used R before. You may want to do some reading on the basics of R. Here's one resource you could use to help you get going.
https://www.computerworld.com/article/2497143/business-intelligence-beginner-s-guide-to-r-introduction.html