Kubuntu cannot build Cantor with its R backend because R is in the universe repository while Cantor (source pacakge: kdeedu) is in main, so cantor cannot depend on R.
Is there any way to build my own Cantor package that include the R backend?
This is a bug.
As a workaround, someone has created a ppa for kdeedu with R support.
Try adding the ppa then installing Cantor.