No problem. This is a working solution, but the exact issue hasn't been identified yet. I'll keep digging into the Armbian build system and see what's up with uboot.
It worked! You need to keep .img files in the same folder as rkdeveloptool folder and run everything as sudo. Thank You very much!!!
andrius-vainorius@DELL-G5:~/rkdeveloptool$ sudo rkdeveloptool db rk3588_spl_loader_v1.15.113.bin
Downloading bootloader succeeded.
andrius-vainorius@DELL-G5:~/rkdeveloptool$ sudo rkdeveloptool wl 0 Armbianserver.img
Write LBA from file (100%)
andrius-vainorius@DELL-G5:~/rkdeveloptool$ sudo rkdeveloptool wl 64 idbloader.img
Write LBA from file (100%)
andrius-vainorius@DELL-G5:~/rkdeveloptool$