伪代码显示未知命令，但是可以正确执行

文章中用到了伪代码，我将网上的内容输入，可以正确执行，但是TexStudio显示‘未知命令’是什么原因？

\documentclass{article}
\usepackage{algorithm}  
\usepackage{algpseudocode}  
\usepackage{amsmath}  
\renewcommand{\algorithmicrequire}{\textbf{Input:}}  % Use Input in the format 
%of Algorithm  
\renewcommand{\algorithmicensure}{\textbf{Output:}} % Use Output in the format 
\begin{document}

  \begin{algorithm}[htb]  
    \caption{ Framework of ensemble learning for our system.}  
    \label{alg:Framwork}  
    \begin{algorithmic}[2]  
        \Require  
        The set of positive samples for current batch, $P_n$;  
        The set of unlabelled samples for current batch, $U_n$;  
        Ensemble of classifiers on former batches, $E_{n-1}$;  
        \Ensure  
        Ensemble of classifiers on the current batch, $E_n$;  
        \State Extracting the set of reliable negative and/or positive samples 
        $T_n$ from $U_n$ with help of $P_n$;  
        \label{code:fram:extract}  
        \State Training ensemble of classifiers $E$ on $T_n \cup P_n$, with 
        help of data in former batches;  
        \label{code:fram:trainbase}  
        \State $E_n=E_{n-1}cup E$;  
        \label{code:fram:add}  
        \State Classifying samples in $U_n-T_n$ by $E_n$;  
        \label{code:fram:classify}  
        \State Deleting some weak classifiers in $E_n$ so as to keep the 
        capacity of $E_n$;  
        \label{code:fram:select} \\  
        \Return $E_n$;  
    \end{algorithmic}  
\end{algorithm}  

\end{document}