Is sierra 10.? Able to upgrade iOS system

That is a late 2013 iMac. It can be upgraded to macOS Catalina 10.15.x but no later.

There have been 5 major macOS releases since Catalina (Big Sur, Monterey, Ventura, Sonoma and the current Sequoia).

See the following from Identify your iMac model - Apple Support

iMac (21.5-inch, Late 2013)

Model Identifier: iMac14,1

Part Number: ME086xx/A, ME087xx/A

Newest compatible operating system: macOS Catalina

You are considering buying a Apple iMac 21.5in 2.7GHz Core i5 (ME086LL/A) All In One Desktop, 8GB Memory, 256GB Solid State Drive, MacOS 10.12 Sierra (Renewed) which may be obsolete now or very soon. This means it cannot have Mac OS updated to current versions of Mac OS and parts may not be available for it. If you are buying this from a reputable dealer such as Macsales.com at least you will have some support. NEVER EVER purchase a used Mac from:

• Ebay
• Nextdoor
• Marketplace
• Local PC store
• Friend or family

Why, more often than not these machines have not been setup correctly per Apple's instructions in What to do before you sell, give away, or trade in your Mac and you are buying someone elses problems. Machines that are not setup following these instructions will NEVER work correctly as they are still associated with the original owner.

I would strongly recommend that if you cannot purchase a new Mac than consider an Apple refurbished Mac which you can find in Apple's Refurbished store at www.apple.com , scroll to the page bottom and look for the link for the Refurbished store. These machines are identical to new, will run current versions of Mac OS, have the same warranty as a new machine, are eligible for AppleCare and can save hundreds of $$$. If you cannot afford one of these navigate to macsales.com which is a trusted seller of used Macs.

That machine is also unsupported. It will not be able to run a supported version of macOS at all.

Unless you need to run 32-bit applications, you are probably best off upgrading to an Apple-Silicon-based Mac.

The screen on that 21.5" Late 2013 iMac is a 21.5" 1920x1080 pixel screen, so while it is of decent quality, it isn't anything special like a 27" 5K Retina iMac, 21.5" 4K Retina iMac, or 24" 4.5K Retina iMac screen.

You may want to look at a 24" M4 iMac – or a M4 or M4 Pro Mac mini paired with a keyboard, mouse, and monitor of your choice.