Target Display Mode Not Working: iMac 27-inch, mid 2011 and MacBook Pro 2017

I've heard of some Keyboards not working...

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display.

• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display.

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

The iMac cannot be running Mojave or Catalina, but there is no such restriction for the MacBook Pro, It can use Catalina.

Nothing, your MBP must have 10.13.6 or earlier installed on it for Target Display Mode to work. To be 100% honest, I strongly recommend that you just buy and external display and connect it to the MBP. Target Display Mode was discontinued and is no longer supported. Return the expensive cables you used to connect the iMac and put that money towards buying an external display. Please remember Macs are computer and are not really intended to be external displays for other devices.

The GOOD news is external displays have come down in price and are VERY simple to set up.