Getting Tag Plus in ... is Protected error

Question

I'm new to Mathematica. I've never used it before. I connected the Mathematica Kernel to VS Code using this extension: https://marketplace.visualstudio.com/items?itemName=njpipeorgan.wolfram-language-notebook

I tried some simple operations form the Documentation: https://reference.wolfram.com/language/ref/Derivative.html

f[x_] := Sin[x] + x^2
D[f[x], x]

But I get this error:

SetDelayed::write: Tag Plus in (x  + Sin[x])[x_] is Protected.

$Failed

(2*x + Cos[x])[x] + Derivative[1][x^2 + Sin[x]][x]

I also tried it in Editor that comes with Wolfram Mathematica. I get the same error there. Can some one explain to me what I'm doing wrong?