This package is now deprecated. qtl2geno, qtl2scan, qtl2plot, and qtl2db have been merged into a single package, qtl2.
Install the new package and remove the old ones:
install.packages("qtl2", repos="https://rqtl.org/qtl2cran")
remove.packages(paste0("qtl2", c("geno", "scan", "plot", "db")))