Use IMac as a display for my MacBook Pro.

Target display mode is a feature that was available only with certain older iMac models.

The feature has been deprecated by Apple and is no longer supported in hardware or software.

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

Having said that, per the link above, only the following older iMac models could be used as an external display for another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.

• The intended target iMac (your iMac 27" 2010) must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.

• Retina display iMacs won’t work as target displays.

• In all cases the source Mac that you're connecting FROM (your 2010 MBP) must have macOS 10.15 Catalina or earlier installed.

I will suggest that if you can't make this work easily, consider that any number of decent monitors can be had today for $100 or less. That would typically include easy setup, no headache and no special ($$) cabling. And these monitors will use less electrical energy than your old iMac. 👍🏽

Both your computer support the now-deprecated Target Display Mode. All you should need is a MiniDisplayPort cable. That is how I connected my 2012 Macbook Pro to a 2010 iMac 27, and the 2010 Macbook Pros have the same ports and my 2012 model.

That worked for my very similar models. Invoking TDM uses keystrokes, not system prefers as anther person asked here this morning. The instructions are in the article that D.I. Johnson linked.

Not possible unfortunately. Apple will recycle it for free at your nearest store, that’s best.

You’re welcome. 🙂