The ASCII character set only accepts values in the range 0 to 127 inclusive: your byte value of 0xF3 - a hexadecimal value equivelant to 243 in decimal - is outside that range and cannot be translated to an ASCII character.
You are trying to read data as text, but the file does not contain the "right data" - I'd suggest you check the file content, and probably read it as binary data instead of text:
Working with Binary Data in Python | DevDungeon[
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