[Nexus 5X Original Android Development] 3ndymion:
Oh, thanks for pointing this out. I experienced this same exact thing. After I flashed the ROM & other things, I went to reboot, & TWRP warned that there is no OS installed. I knew I just installed the OS (Lineage), & it did succeed, so I just ignored it & rebooted it anyway. Lineage loaded up fine. As you seem to know already, that warning message obviously should not be there after you flash a ROM. It is a bug in TWRP. I do not remember if previous versions did the same thing or not, but this version (3.2.3-0) certainly does. That is something that should be brought to the attention of the TWRP developers, if they're not aware of it already.

For anyone new coming into this, you will likely see that scary message too, warning that no OS has been installed. If you just flashed the ROM, & it succeeded, then the OS is definitely there. Don't even worry about the message. Just reboot into the system whenever you're finished, & it will be fine.

On this subject, let's go a little further. Let's think of an imaginary scenario in which there actually is no OS. I haven't actually tried this myself, but I'm pretty sure it should be something like this...

Let's say I decided to go into TWRP recovery & wipe everything clean. I deleted the OS completely. Now I decided to troll myself & boot into the OS, even though it's completely blank. What would happen??? Well, when it leaves TWRP recovery, it will try to boot into the OS & see that nothing is there. The system will probably crash & reboot itself & try booting into the OS again, crash & reboot & try again... bootloop. That's likely the only thing that will happen. The system is simply trying to boot into something, so there shouldn't be any kind of damage or destruction going on, nothing crazy like that to worry about.

As long as you are using TWRP as your recovery, then your recovery is always there. The recovery program resides in its own partition, & you did not delete or wipe clean that partition. So, if something ever goes wrong & you see bootlooping, you can always reboot into the recovery partition & flash whatever you need to flash. With TWRP, that's how it is. That may not be the case with a stock recovery, so take caution if you are using a stock recovery. Again, I haven't actually tried it myself, but it should be something along those lines.

To add another note, for anyone who doesn't know, every computing device has an emergency hardware power cutoff. It is activated by holding down the power button for 10 - 20 seconds. Doing that will completely cut off power to the device. So, if the device crashes, & the OS is completely stuck, there's no way that the software can reboot, or turn off, or do anything, because it crashed. In that case, simply hold down the power button to cut off the power. This is a hardware function that does not rely on any software at all. It is especially important for mobile devices, since they no longer make them so that you can remove the battery whenever you want to.

---------- Post added at 08:37 ---------- Previous post was at 07:53 ----------

It could be the fact that the bootloader on your Nexus 5X is unlocked that's causing it to be not certified. Maybe Google put stricter tests on it since it's their own device, & their own ROM??? But then again, you mention your Nexus 7 devices as well. That's quite strange that 1 is certified, & the other is not. I wonder why that is??? My knowledge is not up to par on this stuff, but those links you posted are a good start.

This is the year I can finally return to online activities, but right now, my time is still limited. All I can do for now is document my own experiences. I managed to get mine certified by going through the steps I described in my 1st long post a couple of pages back. It is still holding. It still shows as certified in Google Play, & I can still see Super Mario Run if I search for "mario", as well as other Nintendo games.

To add a little more, I reflashed my old T-Mobile Samsung Galaxy S4 with a fresh, new, clean install of Lineage 14.1, again with Magisk. In Magisk, both checks for ctsProfile & Safetynet showed as passed. Google Play shows it as certified, & the "mario" search works. In the build.prop file, I can see the security patch date as 2018-12-05. The fingerprint shows 5.0.1, implying that the device is running Android 5.0.1. It is possible that Play thinks that my S4 is running the old 5.0.1 version of Android, & therefore does tests that are less strict. I saw that mentioned in the Safetynet link you posted. Well, that's my experience so far.

[MENTION=4914592]kellrobinson[/MENTION] Hi. Does the Play store show your device as certified in the settings menu??? Also, if you search for "mario", do you see Super Mario Run as the 1st thing that comes up???

---------- Post added at 08:47 ---------- Previous post was at 08:37 ----------

Hi. As far as I know, that is normal behavior. It does that when your phone is encrypted, & you have a security password or pattern set. The system uses your password as the encryption password. When you reboot, the /data partition must be decrypted before it can see anything in there. That is why it stops & asks for your password. If you check in the settings, you should it say that your phone is encrypted somewhere in the security section. Always be sure to use the strongest or longest password that you can always remember.

Having your phone encrypted is a very good thing. I see many people always want to disable encryption, but I think that is a very bad idea. There is almost no performance hit from using encryption on our devices. It makes no sense to forcefully disable it. Just my opinion. :)