Sidecar not working for me with Mac Mini late 2012 and iPad 5th generation.

That is because you need a 2018 Mac Mini and a 6th gen iPad.

see > Use your iPad as a second display for your Mac with Sidecar - Apple Support

Mac using macOS Catalina

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2016 or later, plus iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019
•   

iPad using iPadOS 13

• iPad Pro: all models
• iPad (6th generation) or later
• iPad mini (5th generation)
• iPad Air (3rd generation)