Aquí está el código que intenté, traté de maneras. Básicamente solo me estoy preguntando sobre el permiso denegado parte.

 data <- matrix(c(1,2,3,4), ncol=2)
 write.csv(data,"C:/Users/brian_000/Documents")

Error:

Error in file(file, ifelse(append, "a", "w")) : 
  cannot open the connection
 In addition: Warning message:
 In file(file, ifelse(append, "a", "w")) :
   cannot open file 'C:/Users/brian_000/Documents': Permission denied
   write.csv(data, file="C:/Users/brian_000/Documents")
  Error in file(file, ifelse(append, "a", "w")) : 
 cannot open the connection
 In addition: Warning message:
 In file(file, ifelse(append, "a", "w")) :
  cannot open file 'C:/Users/brian_000/Documents': Permission denied