Im beginner in R and I want to convert my casuarina_genome.gff file to casuarina_genome.gtf. After looking for several web links I couldn't convert it to .gtf. The purpose of .gtf extension is to provide the genome.gtf file into crispRdesignR package to check for off-targets. Kindly help me out to convert the .gff to .gtf.

Maybe this overview helps: https://github.com/NBISweden/GAAS/blob/master/annotation/knowledge/gff_to_gtf.md

You might also be able to use the Bioconductor package rtracklayer for this:

library(rtracklayer)

test_path <- system.file("tests", package = "rtracklayer")
test_gff3 <- file.path(test_path, "genes.gff3")
test <- import(test_gff3)
export(test,"test.gtf","gtf")

Code is from a post at Biostars I once saw.

The gffread utility can convert from gff to gtf quite nicely and is generally useful for other processing on gff files (extracting fast of genes, etc).