Reference the LyX module instead of the LaTeX amsmath package.
\family typewriter
subequations
\family default
- environment, provided by the
+ environment is provided in LyX by the
\family typewriter
-amsmath
+subequations
\family default
- package, is helpful to group logically-linked equations.
+ module (Document > Settings > Modules).
+ It is helpful to group logically-linked equations.
For example, the one-speed one-dimensional integro-differential equation
with
\begin_inset Formula $0\le x\le X$