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Probably because you're asking for 3 uchar values. Look at the lines with .at<Vec3b>. The type between the < and > is the type it returns. In as grayscale image, the type is uchar, but you're asking for a vector of 3 uchar. Try replacing the Vec3b with uchar.
https://docs.opencv.org/4.1.0/d3/d63/classcv_1_1Mat.html#aa5d20fc86d41d59e4d71ae93daee9726
