SD Card Reader

SD cards formatted for cameras may NOT show up on the desktop like an external drive. However, you can access images and videos using the Image Capture utility that came bundled with macOS

Transfer images in Image Capture on Mac - Apple Support

If this is a camera card, the other option is to install whatever image management software your camera makers provides.

First verify the SDcard reader is being recognized by the system (this eliminates a hardware issue) from

>(option key) System Info>Hardware>Card Reader Should see something similar:

The SDcard reader has always been a bit finicky— maybe it will take 100 tries.

Try and blow clean any debris in the slot, come in with varying angles of attack; granted not a huge option here, but the idea is to apply pressure in various ways as you insert the card to initiate a better contact on all the points

About the SD and SDXC card slot on your Mac - Apple Support

An alternative is an external card reader, they are relatively inexpensive. from any office supply store...

one example:

https://www.bestbuy.com/site/camera-memory-cards-usb-drives/memory-card-readers-adapters/abcat0404007.c?id=abcat0404007