iPhone error code 27 problem is yet to be resolved until now? Then, it may be an iOS glitch, which could be corrected by restoring your iPhone under Device Firmware Update (DFU) mode. The method is effective; however, you will end up losing all your data including photos, favorite music albums, and videos. Below is a detailed procedure to restore iPhone in DFU mode using iTunes. If you don't care data loss or you have had a backup, do as follows.
For iPhone 8 and newer:
i: Connect the iPhone to the PC using a legit USB;
ii: Quickly press and release the "Volume Up" button then do the same to the "Volume Down" button quickly;
iii: Hold the "Side" button until the screen goes dark. Now, hold down both "Side" and "Volume Down" buttons for about 5 seconds;
iv: Keep holding the "Volume Down" button while releasing the "Side" button. Nothing displaying on the screen means that the iPhone has been into DFU mode.
For iPhone 7/7 Plus:
i: Connect the iPhone to the PC using a genuine Apple USB cable;
ii: Simultaneously press and hold the "Side" button and the "Volume Down" button;
iii: After 8-10 seconds, release the "Side" button and keep holding the "Volume Down" button. If nothing appears on the screen, the iPhone is in DFU mode.
For iPhone 6S and older:
i: Connect your iPhone to the PC;
ii: Press and hold the "Home" button and the "Lock" button for about 10 seconds;
iii: Release the "Lock" button while holding the "Home" button simultaneously;
iv: If no iTunes logo shows on the screen, the iPhone gets into DFU mode.
Note: However, instead of using the DFU mode solution, you might consider going for a much safer option, one that does not involve data loss.