Could you save the log from the installer in TWRP and post it here?
It is true. S9+ only Samsung boot logo on and off in 2-3 seconds Intervall.
Here is the log:
AROMA INSTALLER version 3.00
(c) 2013-2015 by amarullz.com
ROM Name : ElementalX
ROM Version : 3.03
ROM Author : flar2
Device : Samsung Galaxy S9/S9+
Start at : Thu Jun 25 08:36:40 2020
installing samsung updater extensions
Installing ElementalX...
Extract: /tmp/G960-dtb.patch
Extract: /tmp/G960-oc-dtb.patch
Extract: /tmp/G960-oc-zImage.patch
Extract: /tmp/G960-uc-dtb.patch
Extract: /tmp/G960-uc-zImage.patch
Extract: /tmp/G960-zImage.patch
Extract: /tmp/G965-dtb
Extract: /tmp/G965-oc-dtb.patch
Extract: /tmp/G965-oc-zImage.patch
Extract: /tmp/G965-uc-dtb.patch
Extract: /tmp/G965-uc-zImage.patch
Extract: /tmp/G965-zImage
Extract: /tmp/bspatch
Extract: /tmp/busybox
Extract: /tmp/install.sh
Extract: /tmp/magiskboot
about to run program [/tmp/install.sh] with 1 args
112640+0 records in
112640+0 records out
57671680 bytes (55.0MB) copied, 0.226703 seconds, 242.6MB/s
Parsing boot image: [/tmp/boot.img]
HEADER_VER [0]
KERNEL_SZ [31207448]
RAMDISK_SZ [1143192]
SECOND_SZ [0]
EXTRA_SZ [305152]
OS_VERSION [10.0.0]
OS_PATCH_LEVEL [2020-06]
PAGESIZE [2048]
NAME [SRPQH16A009KU]
CMDLINE [Repacked_by_SoLdieR9312]
CHECKSUM [55992c0d3a0fc24ccc88f542050dab66c2c61bd3000000000000000000000000]
KERNEL_FMT [raw]
RAMDISK_FMT [gzip]
MODEL=
CPU=3
open: /tmp/split_img/kernel failed with 2: No such file or directory
Loading cpio: [ramdisk.cpio]
Loading cpio: [/tmp/split_img/ramdisk.cpio]
Extract [.backup/.magisk] to [.magisk]
Loading dtbs from [/tmp/split_img/kernel_dtb]
open: /tmp/split_img/kernel_dtb failed with 2: No such file or directory
Parsing boot image: [/tmp/boot.img]
HEADER_VER [0]
KERNEL_SZ [31207448]
RAMDISK_SZ [1143192]
SECOND_SZ [0]
EXTRA_SZ [305152]
OS_VERSION [10.0.0]
OS_PATCH_LEVEL [2020-06]
PAGESIZE [2048]
NAME [SRPQH16A009KU]
CMDLINE [Repacked_by_SoLdieR9312]
CHECKSUM [55992c0d3a0fc24ccc88f542050dab66c2c61bd3000000000000000000000000]
KERNEL_FMT [raw]
RAMDISK_FMT [gzip]
Repack to boot image: [/tmp/newboot.img]
HEADER_VER [0]
KERNEL_SZ [0]
RAMDISK_SZ [1143192]
SECOND_SZ [0]
EXTRA_SZ [305152]
OS_VERSION [10.0.0]
OS_PATCH_LEVEL [2020-06]
PAGESIZE [2048]
NAME [SRPQH16A009KU]
CMDLINE [Repacked_by_SoLdieR9312]
CHECKSUM [25556230de2572e46f89fd3b2ea68fd57911c6f4000000000000000000000000]
2836+1 records in
2836+1 records out
1452052 bytes (1.4MB) copied, 0.089693 seconds, 15.4MB/s
Finished!
script result was [1.0]
Installer Sucessfull (Status 0)
End at : Thu Jun 25 08:36:41 2020