I have a iMAC late 2015, 21.5" version 10.11.6. I want to know what is the latest OS version that I can upgrade to, that will run my CS5 creative suit

Looks like Mojave...

Adobe Photoshop CS5 – RoaringApps

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

macOS High Sierra - Technical Specifications

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

macappstores://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Adobe CS5 will run on macOS Mojave (10.14) but you are better off going no higher than High Sierra (10.13.6) on an HFS+ formatted system drive.

Also, check with Adobe first as to whether or not you can still enter a CS5 license code; many vendors have discontinued license services for old software. If you are upgrading macOS where CS5 is already installed it may not matter; but if you intend to do a fresh install of CS5 it could potentially be a problem.

All OS X/macOS up to High Sierra use HFS+ as the format of the system drive. It's also called MacOS Extended (Journaled). So, your El Capitan Mac's drive is already formatted that way.

I mentioned HFS+ because in High Sierra there is an option to use APFS instead of HFS+. CS5 will work better on HFS+. APFS, on the other hand, is required for Mojave and later versions of macOS.