cannot upload pics from camera to iMac using OS Catalina

What make and model is the camera?

If connecting the camera to the iMac using it’s USB cable doesn’t work. Then try removing the camera card from the camera and put it into the iMac’s card reader or into an external USB card reader.

also see > Import photos from a camera or phone using Photos on Mac - Apple Support

Please get remove the SD from the computer, insert into the card reader your iMac has (I'm assuming you have an iMac with a reader slot) and move your photos that way. If your iMac is a newer 24" that does not have a SD card reader, pick up a third party reader and connect it.

Also, please remember we are not mind readers. We have no idea what computer you own (be specific of the model year and size), what version of Mac OS you have installed, and finally what you have attempted. Please read Writing an effective Apple Support Communities Question to get an idea of the information we need from you in order to be really helpful.