Can't downgrade from 5.1.5.0A

Hi guys! I’m stuck on Flyme 5.1.5.0A. Tried to install 4.5.7A and 4.5.6A it says “Firmware corrupt”. Also can’t upgrade new version. Can someone help please? I want to return international flyme because i need bulgarian language. It was on it. My MX4 is from TCCM. I’m worried if i can’t return it back.

Meizu Pro 5

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