Using late 2015 IMAC as display for a new mac mini

None of new Mac's including the Mac mini's, support Target Display Mode.

carefully read > Use your iMac as a display with target display mode - Apple Support

If you have or are thinking about getting a new Mac mini, then you will need an external display.

see > Connect a display to Mac mini - Apple Support

That Mac is too new to support Target Display Mode – and current Macs wouldn't support using it as a Target Display even if it did. It's also too old to be an AirPlay Receiver for AirPlay to Mac.