This message has been encoded by a monoalphabetic function $f(p)=p+b~ \pmod{26}: APHUO~ EGEHP~ PEXOV~ FKEUH~ CKVUE~ CHKVE~ APHUO,$ where we digitize the alphabet by letting $A = 00, B = 01, . . . , Z = 25.$
I want to find the original message. For this purpose, I tried to identify $b$ by finding the most occurring letter in the ciphertext which is E, so E=E+b gives us b=0 and encoding is $f(p)=p$ and decoding function is $f^{-1} (p)= p$. I can not find the original message with my decoding function. Can you please tell me where is my mistake?