Orsdxy

I'm trying to use the 'newcommand' in the environment 'align', for that I created the 'newcommand' rbel that is an 'arrow' with a 'n' below It.
I put all the packages that I'm using.

     documentclass[A4,12pt]{article}

        usepackage{amsmath,amsfonts,amscd,amssymb,amsthm,amsbsy}

        usepackage[english]{babel}

        usepackage[utf8]{inputenc}

        usepackage[T1]{fontenc}

        usepackage{graphicx}

        usepackage{abraces}

        usepackage{upgreek}

        usepackage{mathrsfs}   

        usepackage{dsfont}

        usepackage[hang,small,labelfont=bf,up,textfont=it,up]{caption} 

        usepackage{dsfont,float}

        usepackage{color}

    newcommand{rbel}{

        mbox{;rightarrow_{hspace*{-0.3cm}{small n}};~}

        }

    begin{document}

    begin{align}

   theta_n>1, quad 

dfrac{1}{theta_n^2} rbel 0

    end{align}

    end{document}

That gives these messages

"Missing } inserted. end{align}"

"Missing cr inserted. end{align}"

"Missing $ inserted. end{align}"

"Extra }, or forgotten $. end{align}"

It seems you want an arrow with “n” underneath:

documentclass[a4paper,12pt]{article}

usepackage{amsmath}



newcommand{rbel}{xrightarrow[n]{}}



begin{document}



[

theta_n>1, quad dfrac{1}{theta_n^2} rbel 0

]



end{document}

The error is because you're exiting from math mode when in mbox.

I removed all the unnecessary package; please, take note that the option for A4 paper is called a4paper, not A4.

It works if the mbox is replaced by ensuremath. Furthermore the code causes some warnings. A4 is no valid option for the article class and small is ignored in math mode.

documentclass[12pt]{article}

usepackage{amsmath,amsfonts,amscd,amssymb,amsthm,amsbsy}

usepackage[english]{babel}

usepackage[utf8]{inputenc}

usepackage[T1]{fontenc}

usepackage{graphicx}

usepackage{abraces}

usepackage{upgreek}

usepackage{mathrsfs}   

usepackage{dsfont}

usepackage[hang,small,labelfont=bf,up,textfont=it,up]{caption} 

usepackage{dsfont,float}

usepackage{color}

newcommand{rbel}{

ensuremath{;rightarrow_{hspace*{-0.3cm}{n}};~}

}

begin{document}

begin{align}

theta_n>1, quad 

dfrac{1}{theta_n^2} rbel 0

end{align}

end{document}

Another simple solution for your question using tikz-cd. You can observe from the picture that n is more near at the arrow of same dimension of the lenght of xrightarrow.

Here there is the MWE:

documentclass[12pt]{article}

usepackage{mathtools,tikz-cd}

begin{document}

[

theta_n>1, quad begin{tikzcd}[column sep=.4cm]

dfrac{1}{theta_n^2} arrow[r, "n"'] & 0

end{tikzcd}

]

end{document}