Is my iMac compatible with a Sandisk Ultra Plus

AFAIK, your late-2015 Retina iMac has a built-in SDXC card slot that supports UHS-I cards, which covers most of the SanDisk models people use in cameras. The issue isn’t so much "compatibility" with the computer itself, but rather how the card is formatted or which generation of Ultra Plus you have. Newer cards sometimes ship formatted as exFAT or in a way optimized for cameras, and macOS can occasionally choke if the formatting isn’t clean.

Here’s what I’d suggest you try:

1. First, make sure the card reader slot is clean — a bit of dust can cause misreads.
2. Open Disk Utility (Applications > Utilities) with the card inserted and see if it shows up in the left-hand list, even if greyed out.
3. If it does, try erasing the card and formatting it as ExFAT (best for cross-compatibility with cameras and Macs) or Mac OS Extended (Journaled) if you’ll use it only with macOS.
4. If it doesn’t appear at all, test the card with an external USB SD card reader — sometimes built-in readers on older iMacs won’t recognize higher-capacity UHS-I/II cards reliably.
5. Lastly, confirm the card itself isn’t defective by trying it in your camera or another computer.