Hello RogierSchmit,
Thanks for posting in the Apple Support Communities.
It sounds like Touch ID isn't being presented in System Preferences, although you have a paired keyboard with Touch ID, right?
If we understood this correctly, let's go ahead and connect the keyboard with your iMac if it isn't already. Once that's been done, turn the switch off for about 15 seconds before turning it on. This will be similar to a reset to be sure the connection isn't interrupted, and try checking Touch ID settings after giving some time to connect. For additional details on Touch ID settings, take a look at the following resource: Change Touch ID & Password settings on Mac - Apple Support
To change these settings, choose Apple menu
> System Settings, then click Touch ID & Password
in the sidebar. (You may need to scroll down.)
If you're still unable to set this up, contact Apple Support directly to continue checking this.
Have a great day!