If you want to select the text in the lightbox caption – this is not possible because all mouse events are handled by PhotoSwipe itself for certain actions like dragging the images etc. – therefore it is not possible to select any text in the lightbox caption.
If you think this should be changed, you may add this as a feature request for PhotoSwipe or the PhotoSwipe Dynamic Caption plugin at Github:
https://github.com/dimsemenov/PhotoSwipe/issues
https://github.com/dimsemenov/photoswipe-dynamic-caption-plugin/issues