[OnePlus 7 Pro ROMs, Kernels, Recoveries, & Other D] AlkaliV2:
I hate to say this, but if you are failing to boot I think there is another issue altogether. Maybe a bad download, TWRP version not working well with your device among other issues that we won't really be able to help especially not knowing which model device you have. I'll write up something extensive and you can try it but this ROM is only for the Oneplus 7 Pro (International or Tmo version that has been flashed to International firmware). The Sprint version could potentially not be compatible due to hardware differences so if you have a weird model of the device that could be the initial issue. Now that we've covered that lets talk best case scenario for install. This guide is not aimed at you, it is something I was going to write up for awhile now for all new users so this is just for all users to reference later. If something is incorrect let me know and I'll change it.

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Step 1: MSM back to OOS

Explanation - Switching between ROMs like lineageOS can sometimes change the underlying files on your device. MSM is a sane starting point because we know all the major partitions needed from OOS will be restored and ready for use in CrDroid and other LoS/AOSP variants.

Step 2: Take the OTA to 10.3.4/10.0.7 and Reboot back to OOS when completed (Check Device Info and ensure you are booted into latest Stable OTA release)

Explanation - You can potentially skip this step if you want but I always do this to bring at least one slot (A or B) up to Android 10 and bringing the most recent blobs to the device.

****NOTE: Open Beta is NOT supported as blob use for any LoS or AOSP ROM released for this device so do NOT use Open Beta software for flashing over these ROMs!****

Step 3: Unlock bootloader

Explanation - None needed; you know why this is necessary.

Step 4: Boot to TWRP 3.3.1-79 using the .img file

Explanation - I have encountered issues with 3.4.0.0 variations myself but I find 3.3.1-79 to be solid for all moves to LoS/AOSP ROMs. I recommend this recovery

Step 5: While in TWRP Flash latest Stable OOS (10.3.4) and TWRP 3.3.1-79

Explanation: Easy explanation is that you took the OTA so that one Slot is now 10.3.4, but now the other slot has to also be 10.3.4 for the blobs to be accessible by CrDroid if you take an OTA or install a new version of CrDroid to a different slot.

Step 6: (Optional but recommended) Reboot to recovery, flash latest Stable OOS and TWRP 3.3.1-79 again then reboot to recovery

Explanation: Technically now both slots should have 10.3.4 on them if you took the stable OTA to be on 10.3.4 however just to be extra sure we flash OOS and TWRP again to ensure that both slots are definitely latest Stable and this doubles down on ensuring no OTA upgrade failures would cause issue since we are bit for bit overwriting the needed partitions.

Step 7: From Recovery Run Factory Reset AND Format Data

Explanation: In Android 10 /data is where your app data exists so it will be cleared by the factory reset. The "Format Data" step is to wipe the Internal memory that can get in the way of booting because of encryption and differences between Android security patches in OOS and AOSP/LoS.

****NOTE: DO NOT WIPE System, Vendor, or any partitions besides /cache, /data and Format Data! If you do, your phone will not boot and the only recovery method is Fastboot flashing or MSM Tool to boot again!****

Step 8: Reboot to recovery and now copy the ROM, TWRP and Magisk to your internal memory card

Explanation: When you format data you lose access to your internal memory even if it seems like it is still there. Until you reboot and see the "TWRP" folder in internal storage you do not have access to that partition so take care to check when rebooting recovery that you have access or your files won't flash reliably in this condition.

Step 9: Flash CrDroid to your device (Gapps are included now) and flash TWRP 3.3.1-79 also

Explanation: CrDroid now contains your Gapps so there isn't a need to reboot and flash Gapps. However if you choose to use the version that does NOT contain Gapps then you must reboot to recovery after flashing TWRP and flash Gapps and Magisk at that point (if you want Magisk of course).

Step 10: (Optional) Reboot to Recovery to Install Magisk

Explanation: If you want root you must reboot to the new slot to install Magisk into the correct boot image.

Step 11: Reboot to System

Explanation: Time to boot the newly installed ROM and it should boot successfully. However, if it bootloops there is a chance your data format failed so go back to recovery and format data again just to be sure. If that doesn't fix it there is another issue in place and that should be addressed in the forums.

Step 12: DO NOT RE-LOCK YOUR BOOTLOADER

Explanation: I don't care if the ROM is signed with Private keys, you aren't rooted or if you are using LoS recovery instead of TWRP--relocked bootloaders with third-party ROMs typically spell disaster. Even if it boots you are setting yourself up for issues later on so we do NOT recommend re-locking the bootloader.