tblr environment in tblr environment

I don't think that you can nest tblr environments, because they seem to do global assignments.

You might prepare the inner one separately, but you have available much better tools for math alignments.

\documentclass{article} \usepackage{tabularray} \usepackage{amsmath} \newsavebox{\tblrbox} \begin{document} Nesting \texttt{tblr} can be made to work \[ \sbox{\tblrbox}{$\begin{tblr}{|cc|} x-1 & -3 \\ y+3 & 4 \\ \end{tblr}$} \begin{tblr}{cr@{\ }c@{\ }l} & \det( \overrightarrow{AM};\overrightarrow{AB}) &=& 0 \\ \iff& \usebox{\tblrbox} &=& 0 \\ \iff& 4(x-1) + 3(y+3) &=& 0 \\ \end{tblr} \] With \texttt{array} it seems to work \[ \begin{tblr}{cr@{\ }c@{\ }l} & \det( \overrightarrow{AM};\overrightarrow{AB}) &=& 0 \\ \iff& \begin{array}{|cc|} x-1 & -3 \\ y+3 & 4 \\ \end{array} &=& 0 \\ \iff& 4(x-1) + 3(y+3) &=& 0 \\ \end{tblr} \] However, with \texttt{amsmath} alignments it's even better and easier \[ \begin{alignedat}{2} && \det( \overrightarrow{AM};\overrightarrow{AB}) &= 0 \\ &\Longleftrightarrow\quad& \begin{vmatrix} x-1 & -3 \\ y+3 & 4 \\ \end{vmatrix} &= 0 \\ &\Longleftrightarrow\quad& 4(x-1) + 3(y+3) &= 0 \end{alignedat} \] \end{document}