bootstrap 4 carousel caption is not visible on small devices

Here's what you ought to do:

1. Remove class d-none.
2. Remove class d-md-block.

associated with the carousel-caption class of Bootstrap carousel.

Remove the classes d-none d-md-block from all of the carousel-caption classes, the class d-none gives the caption a display of none and the class d-md-block gives it a display of block for md and above screens.

Note: you may need to adjust the text size on smaller screens to make everything fit if you have a lot of text in your captions.