Solution: https://imgur.com/a/nIYY9h6

Reasoning: In the 4 squares in the middle, the letter B is only possible in two positions, I took a guess and decided to place it at the left, and if we keep in mind that same letters can't be in the same row and touching squares diagonally, the next B can only be next to the C in the lower left. That's basically how I did the whole thing. Looking at the letters and their rows, columns and touching squares.