Error(s) in re-building vignettes:
  ...
--- re-building ‘checkpoint.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
Warning in httpsSupported() :
  cannot open the connection to 'https://mran.microsoft.com/snapshot'
Warning in httpsSupported() :
  cannot open the connection to 'https://mran.microsoft.com/snapshot'
Warning: Unable to reach MRAN root at http://mran.microsoft.com/snapshot/
Warning: unable to access index for repository http://mran.microsoft.com/snapshot/2017-04-01/src/contrib:
  cannot open URL 'http://mran.microsoft.com/snapshot/2017-04-01/src/contrib/PACKAGES'
--- finished re-building ‘checkpoint.Rmd’

--- re-building ‘managing-checkpoint-archives.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.

checkpoint: Part of the Reproducible R Toolkit from Microsoft
https://mran.microsoft.com/documents/rro/reproducibility/
Unable to connect to MRAN. Skipping some date validations.
Warning in httpsSupported() :
  cannot open the connection to 'https://mran.microsoft.com/snapshot'
Warning in httpsSupported() :
  cannot open the connection to 'https://mran.microsoft.com/snapshot'
Warning: Unable to reach MRAN root at http://mran.microsoft.com/snapshot/
Scanning for packages used in this project
- Discovered 1 packages
Warning: unable to access index for repository http://mran.microsoft.com/snapshot/2015-04-26/src/contrib:
  cannot open URL 'http://mran.microsoft.com/snapshot/2015-04-26/src/contrib/PACKAGES'
Packages not available in repository and won't be installed:
 - darts
All detected packages already installed
checkpoint process complete
---
Quitting from lines 150-154 (managing-checkpoint-archives.Rmd) 
Error: processing vignette 'managing-checkpoint-archives.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘managing-checkpoint-archives.Rmd’

SUMMARY: processing the following file failed:
  ‘managing-checkpoint-archives.Rmd’

Error: Vignette re-building failed.
Execution halted