As the name goes, “SU occupied” refers to the SU files in your phone is already occupied by other software, such as Kingroot. When running SuperSU, the app automatically checks if the system’s SU file is occupied by other software. If so (and if unable to replace automatically), then the notification of “SU occupied” will appear.
To resolve this issue, SuperSU need to be installed via side-loading from custom recovery. The process is very similar to re-rooting your phone.
First, find out which software is currently occuping your SU.
If the software can be removed along with the SU (usually can't, sad!), then simply delete the software. Otherwise, download SuperSU.zip and a custom recovery to reroot your phone.