How can I center my circuit?

I would redraw your circuit scheme as follows:

\documentclass[11pt]{article}
\usepackage[letterpaper, 
            vmargin=2cm, hmargin=2.5cm]{geometry}
\usepackage[american,siunitx]{circuitikz}

%---------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

\begin{document}
    \begin{figure}
    \centering
    \ctikzset{bipoles/length=1cm, nodes width=0.08}
\begin{circuitikz}
\node[op amp] (oa1) {};
\draw   (oa1.out)   to [R=1<\kilo\ohm>,*-*] ++ (2,0)
                    node[op amp,anchor=-] (oa2) {}
        (oa1.-)     -- ++ (-1,0)                coordinate (aux1)   
                    --  ++ (0,1)                coordinate (aux2)
                    to [R=1<\kilo\ohm>] ++ (2,0) 
                    --  (aux2 -| oa1.out) -- (oa1.out)
        (oa1.+)     -- ++ (0,-0.5) node[ground] {}
%
        (aux1)      to [R=1<\kilo\ohm>,*-] ++(-3.5,0) node[left] {$V_1$}
                    to [V_=3<\volt>] ++ (0,-1.5)
                    node[ground] {}
        (aux1)      -- ++ (0,-1)
                    to [R=1<\kilo\ohm>] ++(-2,0) node[left] {$V_1$}
                    to [V_=6<\volt>] ++ (0,-1.5)
                    node[ground] {}
%%
        (oa2.out)   to [short,  -*] ++ (1,0)    coordinate (aux3)
        (oa2.-)     |-  ++ (1,1)                coordinate (aux4)
                    to [R=2<\kilo\ohm>]  (aux4 -| aux3) -- (aux3)
                    to [short, -o] ++ (1,0) node[right] {$V_o$}
        (oa2.+)     -- ++ (0,-0.5) node[ground] {}
%%% suply
        (oa1.up)    |- ($(oa2.up)+(0,2)$)       
                    -- ++ (4,0)                 coordinate (aux5)
        (oa2.up)    -- (oa2.up |- aux5)
        (aux5)      to [V_=15<\volt>,-*]    (aux5 |- oa2.out)
        (oa1.down)  |- ($(oa2.down)-(0,2)$)
                    -- ++ (4,0)                 coordinate (aux6)
        (aux5 |- oa2.out)   to [V_=-15<\volt>]    (aux6)
        (aux5 |- oa2.out)   -| ++ (1,-0.5) node[ground] {}
    ;
\end{circuitikz}
    \end{figure}
\end{document}

which after compilation gives:

(red lines mark text width)

In above MWE are in comparison to yours changed:

• size of bipoles are reduced
• used is siunitx notation for units
• all circuits elements are positioned relative to neighbors one and slightly reduced
• image scaling is omitted
• also is omitted unknown circuitikz option H

Edit: Corrected is scheme. Now is as OP provide it in question.

Remove centering and add the following line for \hspace -- rest no change -- your figure will always extend into margins being large

\begin{figure}
\hspace{-2cm}
........
\end{figure}