New answers tagged os
From what I can read on the image you posted it looks like your device has 4.4.4 not 5.1.1, and possibly a custom rom rather than the stock one (it looks like omni_hammerhead/hammerhead: 4.4.4 in the screenshot). If you have a custom ROM you will not be able to apply an OTA update as they will only work for the stock ROM. Your best bet might be to flash a ...
Simply, follow the steps from the link you posted, if you already tried that but still same, you can always flash the stock ftf file for your phone using flashtool, and then you can install cm12 again if you want with the same steps, Sony Xperia P LT22i FTF: Download #4.1.2 Flashtool: Download #latest version Guide for using FT: HERE
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