The example code in section 7.3 _Referencing sub-figures without sub-captions_ of the documentation has a `\begin{subcaptiongroup}` but a non matching `\end{subcaptionblock}` in the CTAN version 1.6b 2023-07-28 and thus does not compile. I would highly recommend correcting this using either `subcaptiongroup` or `subcaptionblock`.