I upload photos to Lightroom Classic.
I believe you mean you Import photos into Lightroom Classic.
I eject and remove the card reader. When I go to Development (of the Photograph), a tag comes up saying "photo not found."
You have imported the photos using ADD (which means they are left on the camera card), and since you have removed the card from the card reader, LrC can't find the photos. You really ought to be importing using COPY which should make a copy of the photos on one of your hard disks and then LrC can find them.