Rcpp code throwing error when knitting in Rstudio

Im trying to run the following rmarkdown code:

https://docs.google.com/document/d/164VZx3_cZ4tBa1Um39VH1cQ4zqRuCvpCTj-B50Vq2gk/edit?usp=sharing

But Im getting the following error when I knit the code:

Quitting from lines 26-47 (demo.Rmd) 
Error in (function (file = "", code = NULL, env = globalenv(), embeddedR = TRUE,  : 
Error 1 occurred building shared library.
Calls: <Anonymous> ... block_exec -> in_dir -> engine -> do.call -> <Anonymous>
In addition: Warning message:
No function return type found for Rcpp::export attribute at file1af863722d6.cpp:4 
Execution halted `

When I replace {Rcpp} with {rcpp} I get to run it but its not giving the output. How do I get it to run and give the output?