Hello,
I'm following the RNA-seq workflow : gene-level exploratory analysis and differential expression.

in this step, the error appear,

> library("GenomicFeatures")

> gtffile <- file.path(dir,"Homo_sapiens.GRCh37.75_subset.gtf")
> txdb <- makeTranscriptDbFromGFF(gtffile, format="gtf")
Error in nchar(attrs) : 
  could not find symbol "keepNA" in environment of the generic function
In addition: Warning message:
'makeTranscriptDbFromGFF' is deprecated.
Use 'makeTxDbFromGFF' instead.
See help("Deprecated") 
> txdb <- makeTxDbFromGFF(gtffile, format="gtf")
Error in nchar(attrs) : 
  could not find symbol "keepNA" in environment of the generic function.

  I am a newcomer trying to analysis RNASeq data. Can someone help please!

See C: could not find symbol "keepNA" in environment of the generic function, but I think you should be able to solve this with biocLite("S4Vectors"), which will install version 0.6.2. If not, please provide the output of sessionInfo() after the error occurs.