Because only single-word type name could be used for this kind of explicit type conversion.
A single-word type name followed by a braced-init-list is a prvalue of the specified type
designating a temporary (until C++17)whose result object is (since C++17)direct-list-initialized with the specified braced-init-list.
unsigned char is not a single-word type name, while char is. And this is true for functional cast expression too, that’s why ('p') doesn’t work either.
As the workaround, you can
using uc = unsigned char; // or use typedef
const auto ch = uc{'p'};
Or change it to other cast styles.
const auto ch = (unsigned char) 'p'; // c-style cast expression
const auto ch = static_cast<unsigned char>('p'); // static_cast conversion