[Samsung Galaxy J7 (2017) Questions & Answers] thethirdperson:
Hello. Sorry for the long reply ahead. I have the same problem of wanting to use 64 bit apps in my J7 pro (SM-J30G/DS). It's my first time doing a custom rom as well and I followed all the instructions detailed here, but things did not go well for me.

The only step I missed is copying the backup to my laptop because for some reason my device is not letting it be copied. Nevertheless, I proceeded to install the custom rom since I was hoping to use the stock rom in sammobile.com instead as backup.

However, I keep getting both the error 7 and "failed to mount /preload (invalid argument)" when installing crdroid. I tried numerous ways to fix it but nothing happens. The following are what I have already tried:

- Install older versions of crdroid (all versions causes error 7)

- Remove assert in "updater-script" (tried it in 6.9 and 6.7 versions but still get error 7)

- Install latest versions of twrp (I cannot find the latest version of twrp for j7y17lte aside from what's here, so I went to other sites that offer twrp for J7 pro (2017). I found j7xelte and herolte but they don't work, so I used given twrp here again)

I can't fix the error and the install was always unsuccessful, so I decided to use another custom rom which is HavocOS. I followed the instructions but I still get the error 7 when installing project treblue. I tried removing assert again but the error is still there. I researched more but I can't find any more solutions to this problem or what's causing it.

One thing to note is that the create vendor zip was installed successfully (but with failed to mount/preload) and only the custom roms receive error 7. My research about error 7 said that it happens when the device is incompatible with the roms to be installed, but I'm pretty sure that my device is J730G/DS which should work fine since it worked with others here that have the same device. If my device was incompatible, then why does the twrp and create vendor work fine? Is it because my phone was not rooted? I saw that a lot of custom roms require root before installing, but it was not mentioned in the instructions so I did not root my phone.

Another problem is that I can't revert to my stock rom anymore. I found my stock rom in sammobile and proceeded to flash it using odin, but then my phone is stuck in samsung logo after rebooting. I tried leaving it for 8 hours to see if it will load but nothing happened. I can still access the recovery so I reinstalled twrp and would try to reinstall my stock rom.

Any help would be appreciated. Thank you!

EDIT: After reinstalling twrp, I tried flashing the create vendor and crdroid again and for some reason, the installation was now successful. But after rebooting to system, it was stuck in the crdroid logo. I then went to twrp again, formatted the data, then installed HavocOS and to my surprise, it was successful! There are no problems with boot and I am currently able to use the rom smoothly.

Problem is fixed since I am able to use 64 bit apps, but if you guys know what happened and what steps I did wrong, please tell me! I am still confused with what the problem is and I don't want to experience the same problem when I update my rom. Any help would be appreciated. Thank you very much!