Why GF(256) matrix only have zero and one number?

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If you want to work mod 256, you should work with Integers(256), not GF(256). (The first of these is the ring Z/256, the second is the field with 256 elements.)

sage: matrix(Integers(256), [1,2,3,4])
[1 2 3 4]
sage: matrix(Integers(256), [254, 255, 256, 257])
[254 255   0   1]

(edited to add the second example)

IntegerModRing(256) is a synonym for Integers(256), by the way.

This is not a matrix issue. The field K=GF(256) has characteristic 2, which result do you expect for K(3) ?