math/invalid-label

A Label has a character other than letters, digits, :, _, . and -. A Label becomes an id and a link’s #fragment exactly as written, so it is kept to characters that need no escaping.

Example

$$\begin{equation} E = mc^2 \label{eq energy} \end{equation}$$

Fix

Use the Label the help suggests, here eq-energy, and change its references to match.