Use a iMac as an external screen for my MacBook Pro

Hello, duchess.

I don't believe Apple supported that iMac as an external (target) display. 

Some things to know about Target Display mode:

Apple has not supported Target Display mode out of the box on any iMac since 2014, nor on any macOS newer than 10.13 High Sierra.

Only a small handful of iMacs were ever supported to target display mode:

"Note: Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.”

For the intended target (the iMac being used AS a display):

"Target display mode requires macOS High Sierra or earlier. It does not work with macOS Mojave or later.“ (Catalina)

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

Use your iMac as a display - Apple Support

If you search the internet, however, you might find some unsupported third-party solutions. For example: 

Turn any Mac or iPad into a second display - lunadisplay 

You're welcome.

(Sorry about mangling your name before. Stoopid autocorrect!)