Here is an alternate way of structuring the definition of your function that avoids the use of If completely by employing a combination of conditional expression evaluation and argument pattern matching:
j[x_, n_, l_] := 0 /; l < n
j[x_, l_, l_] := x^l
j[x_, n_, l_] := (1-x)/(1 + x^l)*(Sum[j[x, n-1-i,l]*x^i, {i,0,l-1}]) + x^l/(1+x^l)
And plotted:
Plot[j[x, 7, 7], {x, 0, 1}]
