can I connect an Imac screen to a mac mini?

Not with Apple's now defunct Target Display Mode. Both devices are too new.

Third-party solutions to force your iMac to work as a monitor are likely be laggy and cost money. Luna Display is US$79 and the web site includes this advisory:

Your best results will be by using a free-standing external monitor. It will be easier to set up and use far less power that a fully functional iMac 27.

+1 Nope! Please read Target Display Mode which has been dead for many years. Your Mac mini requires an external display, iMacs are not external displays they are computers! You have a lot of options available to you in all price ranges. All will be:

• Much more reliable than Target Display Mode ever was
• Easier to setup
• Easier to maintain
• Have no need for expensive cabling

No.