Why I get "the requested layer 'background' is not part of the list" error if it is part of the list?

Question

The following code gives me the error:

! Package pgf Error: Sorry, the requested layer 'background' is not part of the
 layer list. Please verify that you provided \pgfsetlayers and that 'background
' is part of this list.

But background is in \pgfsetlayers{background,main}. What am I doing wrong?

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\usepgfplotslibrary{fillbetween}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main} 
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    axis x line=center, 
    axis y line=center, 
    ticks=none
    ]
    \addplot[color=red,domain=-2:2,name path= mycurve]{x^2};
    \addplot[domain=-2:2,name path= myline]{x-1};
\begin{pgfonlayer}{background}
    \addplot[red!10] fill between [of=myline and mycurve, soft clip={domain=-1:1}];
\end{pgfonlayer}
\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

OK, I see meanwhile Stefan Pinnow already showed what the issue is. You need to use set layers or axis on top (or something of that sort) and then draw the filling on the desired layer, see section 4.27.2 of the pgfplots manual. The desired answer seem to be

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\usepgfplotslibrary{fillbetween}
\begin{document}
\begin{tikzpicture}
\begin{axis}[axis on top,
    axis x line=center, 
    axis y line=center, 
    ticks=none
    ]
    \addplot[color=red,domain=-2:2,name path= mycurve]{x^2};
    \addplot[domain=-2:2,name path= myline]{x-1};
    \addplot[red!10] fill between [of=myline and mycurve, soft clip={domain=-1:1}];
\end{axis}
\end{tikzpicture}
\end{document}

Here, as kindly pointed out by Stefan Pinnow, the fill between triggers automatically that the shaded area is on the pre main layer.

If you replace axis on top by set layers, you'll get

More options for layers are discussed in section 4.27.2 of the pgfplots manual. These additional options might be relevant if you want to draw several overlapping areas (and/or contours) in a certain order, and you cannot just order the various \addplots to give the desired ordering.

Answer 2

I had a similar problem when trying to draw something on the background layer behind an axis: with

\pgfdeclarelayer{background}
\pgfsetlayers{background,main} 
...
\begin{axis}[]
\begin{pgfonlayer}{background}

I got the error Sorry, the requested layer 'background' is not part of the' is part of this list.

After studying section 4.27.2 of the PGFPLOTS manual, I found that PGFPLOTS creates its own default layers for axis environments: axis background, axis grid, axis ticks, axis lines, axis tick labels, main, axis descriptions, axis foreground.

Thus,

\begin{axis}[]
\begin{pgfonlayer}{axis background} % instead of background

did the trick for me.