I am trying to modify Oreo rom for some vertical screen Android head unit. Its name is 'px6.zip' for rk3399 platform. I have successfully unpacked the rom, but could not get it to flash after repacking. At first, I thought it's some mistakes in the process. However, I tried just modifying the zip without unpacking and repacking, also tried signing the zip after that. However, I always got error msg when trying flashing it, "not a valid update package". Why is that? Is it that I was doing something wrong or the rom was made with some algorithm with checksum or something? Any insights on getting it working are much appreciated.