Yes, double check if OEM is still unlock after Heimdall step. You can’t make it work with adb sideload, sideload is a part of TWRP, that’s why your attempt failed. You really need to check OEM unlock and install TWRP 1st, else nothing can work. Like @0x7C0, i’ve done it on a Galaxy S9, not a S9+. Hope it works for you