Unable to connect MacBook Pro to a Dell monitor

It sounds like you are trying to use an Apple Thunderbolt 3-to-2 adapter to a plug a Dell monitor that has a USB-C (DisplayPort) input into a Thunderbolt 1 port on a 2012 MacBook Pro.

I'm assuming that this Dell monitor doesn't require or understand Thunderbolt. Most third-party monitors do not. I'm also assuming that there is no powered Thunderbolt 3 dock between the Thunderbolt 3 side of the TB 3-to-2 adapter, and the monitor.

In that case, this is not going to work. The Thunderbolt 3-to-2 adapter is bidirectional, but it only knows how to translate Thunderbolt. It does not know how to pull apart a Thunderbolt signal to

• Make its Thunderbolt 2 side act like a Mini DisplayPort
• Make its Thunderbolt 3 side act like a USB-C port that has all of the blades (USB, DisplayPort, Thunderbolt)

The Thunderbolt 1 port on your Mac does have the ability to double as a Mini DisplayPort – and many of the Dell monitors that have USB-C (DisplayPort) inputs also have plain DisplayPort and HDMI ones. If your monitor has a plain DisplayPort input, you could try connecting your Mac to the monitor using a Mini DisplayPort to DisplayPort cable.

Make sure that the thunderbolt ports are not damaged.

I have a 2012 mac too.

Also maybe try using Open Core Legacy Patcher to upgrade to a later version of MacOS.