Inside a module you need to put ; between your expressions. To create a so-called CompoundExpression
ClearAll[coeffs]
coeffs[n0_] := Module[{n = n0, c},
c = Table[0, {i, n + 1}, {j, n + 1}];
Do[
c[[l + 1, 2]] = 20
,
{l, 0, n}
];
c
];
Moreover, it is a good idea to make c a local variable, finally you need to return something. That can be done by just putting c at the end (without semicolon). Or explicitly do so using Return.
