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.
ZIP: Detect AVB signature on input boot image, and sign output image accordingly (force with SIGNBOOTIMAGE, custom keys in /tmp/avb)
ZIP: Samsung/7.0+: if /data is not currently encrypted, disable encryption (unless KEEPFORCEENCRYPT or REMOVEENCRYPTABLE are set)
ZIP: Detect TWRP UI slot switch (Pixel/XL: 3.1.0-RC2+)
ZIP: Further improve /system device detection
ZIP: Support compiling sepolicy from split CILs
ZIP: Kernel patch: support LZ4 compressed kernels (new format only)
ZIP: Mount /vendor
ZIP: Add /vendor/lib to LD_LIBRARY_PATH
ZIP: Initial Pixel+ODP2 support
ZIP+APK: Fixes in boot partition detection
sukernel: Allow cpio-add of 0-byte files
sukernel: Add dtb (in-kernel only) related features
suinit: Improve logging
suinit: Support pre-mounting /system and /system_root (ODP2+)
sukernel+suinit+ZIP: Reduce system_root footprint by dynamically importing entries at boot
supolicy: Adjustments for ODP3