There are errors like in the picture, please help. I don't understand what I'm doing wrong(

f[x_] = x^4; a = -1; b = 1; L = 1; 
gr = Plot[f[x], {x, a, b}, PlotRange -> {{a, b}, {-0.2, 1}}, 
   PlotStyle -> {Hue[0.02], Thickness[0.007]}];

a0 = 1/L \!\(
\*SubsuperscriptBox[\(\[Integral]\), \(-1\), \(1\)]\(f[
      x] \[DifferentialD]x\)\);
a[n_] = 1/L \!\(
\*SubsuperscriptBox[\(\[Integral]\), \(-1\), \(1\)]\(f[x]*Cos[
\*FractionBox[\(n*\[Pi]\), \(L\)]*x] \[DifferentialD]x\)\);
b[n_] = 1/L \!\(
\*SubsuperscriptBox[\(\[Integral]\), \(-1\), \(1\)]\(f[x]*Sin[
\*FractionBox[\(n*\[Pi]\), \(L\)]*x] \[DifferentialD]x\)\);
\[CapitalPhi][x, k] = a0/2 + \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(n = 1\), \(k\)]\((a[n]*Cos[
\*FractionBox[\(n*\[Pi]*x\), \(L\)]] + b[n]*Sin[
\*FractionBox[\(n*\[Pi]*x\), \(L\)]])\)\) // N;
\[CapitalPhi]1 = a0/2 + \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(n = 1\), \(1\)]\((a[n]*Cos[
\*FractionBox[\(n*\[Pi]*x\), \(L\)]] + b[n]*Sin[
\*FractionBox[\(n*\[Pi]*x\), \(L\)]])\)\) // N;

gr1 = Plot[\[CapitalPhi]1, {x, a, b}, 
  PlotStyle -> {Hue[0.1], Dashing[{0.03}], Thickness[0.01]}]

I'm a beginner, I was given a lab, I copied exactly the code from the picture that the teacher sent us, but my code does not work like his. Unfortunately, I can't ask him about my mistake, so I'm asking here