Apple’s TB2-TB3 adapter will not let you attach just any USB-C device. It will let you attach TB3 devices. A USB-C device that does not support TB (as I suspect is the case for your external SSD) will not work attached to a Mac’s TB2 port despite TB3 using the same physical connector as USB-C. The problem in a nutshell is that Mac TB3 ports can speak USB, but TB2 ports cannot. And your disk can only speak USB, not TB. So you need to use a USB port to attach that SSD.
In order to connect your SSD to the USB ports on your 2015 iMac, what you need is an adapter from the Mac’s USB-A to the SSD cable’s USB-C.
Often times more convenient is just to get the right cable all the way from the Mac to the disk. Depending on type port on the disk, that could be any one of these: