How to connect iMac to a MacBook Pro
Can I connect my iMac 24" to a MacBook Pro to use as a second display?
MacBook Pro 15″, OS X 10.10
Question marked as
Best reply
No. Target Display Mode requires an iMac introduced between 2009 and 2014. Additionally, Target Display Mode has been deprecated and only works on Macs that are running macOS High Sierra or earlier.
Use your iMac as a display with target display mode - Apple Support