Ulefone Note 18 Ultra: Can't install system update (Install Error 20)

I've had this issue for a while now. I recently had a virus on this phone, so I did everything I could to remove it just in case it was interfering with the update.

After confirming the virus was completely gone, I performed a factory reset and attempted the system update again. The update fully downloaded, and the installation took about 15–20 minutes, but then it gave me an "Install Failed: Error 20" message.

Before anyone suggests it, my phone is not rooted. I've already tried all the standard troubleshooting methods that don't involve having a rooted device.

Any help or advice would be greatly appreciated. Thanks!