How to start equation environment inside a custom environment?

Question

Definition of env

\newenvironment{env}{
\setlength{\parindent}{0pt}%
\obeylines}

When I try to insert a equation it gives me an error.

\begin{env}
\begin{equation*}
    \sin{x}
\end{equation*}
\end{env}

! Missing $ inserted. $ l.68 \begin{equation*}

? ! Display math should end with $$. ^^M l.68 \begin{equation*}

? ! Missing $ inserted. $ l.69 \sin {dwa} ? ! Missing $ inserted. $ l.69 \sin{dwa}

?

! LaTeX Error: Bad math environment delimiter.

See the LaTeX manual or LaTeX Companion for explanation. Type H for immediate help. ...

l.70 \end{equation*}

? ! Missing $ inserted. $ l.70 \end{equation*}

? ! Display math should end with $$. \endgroup l.70 \end{equation*}

?

If I begin{equtation} outside of the env environment it doesn't give me and error. If I am unable to use env what can I do to achieve the same result?

edit

1.1    Definitions
reward = r_t at timestep t 
behaviour policy =π=P(a|s)
observation = s 
set of legal game actions =A={1,...,K}
action = a
optimal action-value function = ~~

I'm not writing sentences therefore I need to \linebreak.

Answer 1

\obeylines makes every line an end of paragraph and you can not end a paragraph in math mode, what would you want \obeylines to do in display math?

The error is unrelated to the env environment, you get the same error from

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\obeylines
\begin{equation*}
    \sin{x}
\end{equation*}
\end{document}

You could remove the \obeylines (and add back the missing end environment code)

\newenvironment{env}{
\setlength{\parindent}{0pt}%
}
{}

But display math should never be at the start of the paragraph (TeX adds a spurious blank initial line in that case) so this environment could not be used in many contexts.