Hello VentureProd.
Thanks for reaching out to Apple Support Communities. We understand that your card reader is not detected when connected to your MacBook Pro. We will be glad to assist.
Let's have you take a look at the following article to help with SD and SDXC card slot on your Mac. Make sure to take a look at the section titled "If your card doesn’t mount".
About the SD and SDXC card slot on your Mac
Also, are you inserting the card directly into the SD slot or are you using an adapter? If you're using an adapter, the following resource could shed some light on the issue you're having as well.
About the Apple USB-C Digital AV Multiport Adapter
Cheers.