after converting raw counts to cpm from data file using command

lcpm<-cpm(y, log=T)
head(lcpm)

gives first row with unique numeric ID which denotes gene names from data file. How to export name of genes into *.csv file but with name of genes ?

write.csv(highly_variable_lcpm, row.names=y2$genes, file = "highly_variable_lcpm_genes.csv")

I get error message as below

Error in write.table(highly_variable_lcpm, row.names = y2$genes, file = "highly_variable_lcpm_genes.csv", : invalid 'row.names' specification

How to solve the problem ? THx

Read ?write.csv.

row.names: either a logical value indicating whether the row names of
          ‘x’ are to be written along with ‘x’, or a character vector
          of row names to be written.

The argument needs to be a character vector. y2$genes is a data.frame, hence the error.