Can I use my iMac OS 10.11.6 as a display for my new Mac Mini M2 Pro.

The very restrictive hardware and OS requirements for Target Display Mode (TDM) are laid out in this support article:

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

Only a small sub-set of iMac model by year (we don't know your iMac's year) could act as a monitor for another Mac BUT the big stumbling block for you is that no Mac introduced after 2019 can use a TDM-supported iMac as a monitor.

Apple started the deprecation of TDM in late 2014 and threw the last shovel of dirt in its face in erly 2020.

You can establish a screen sharing for your Mac and Mac Mini.

System -> Library -> CoreServices -> Applications -> Screen Sharing is an app which can connect to any LAN IP address that has Screen Sharing enabled over WiFi or ethernet router.

While not as fast as Target Display Mode, it does give you a virtual window on either computer from the other computer.

When the two machines are on both on the internet but one is remote of the other, http://www.anydesk.com/ can bypass the WiFi router and connect the two machines. Someone on the other machine ought to confirm connection for sake of security, and you can verify by phone when it is safe to connect.

sorry, no. the Mac mini is too new to use target display mode.