If your iMac does not detect the SSD in the USB-C Slim Dock for 24" iMac, the SSD most likely needs to be formatted.

1. Open Disk Utility on your iMac.
2. Select the SSD from the list.
3. Click Erase.
4. Name your drive.
5. Select APFS or Mac OS Extended as the format, then erase the drive.

Erasing removes all data on the SSD and formats it so your iMac can access it.

Also make sure the SSD is supported: M.2 NVMe or SATA, size 2242, 2260, or 2280, up to 2TB, and without a heat sink.