Firstly, a warm welcome to Mac-Forums.
May I just clarify: you say your iMac is running 16.2.6? I think you mean that yours is a 4K 21.5", Late 2015 iMac.
When we talk about the Mac "running" something we are referring to the Operating System (OS).
So it would be helpful if you gave us that info, please.
As regards your problem:
It could be a faulty cable, but you've tried a new one - I assume it's the appropriate cable - best of all a Mac cable rather than a third party one? Third party cables can be unreliable.
It could be a problem with your iPod; they are very old, having been discontinued a good few years ago.
So, for the moment, let's have your OS and some detail re the cable, please.
Ian