Failed installation of macOS Sierra on 2016 iMac after second-hand purchase

Buying a used Mac can be a real nightmare, if the previous owner did not properly prepare it for resale.

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If it did not startup to the welcome screen when you first turn it on, then you need to return that one to the seller and buy one that is properly prepared for resale.

See if booting Apple Diagnostics reports any errors: Use Apple Diagnostics to test your Mac - Apple Support

Apple Diagnostics can usually spot hard errors, but can (or will) miss transient errors.

Buying used Macs from untrusted sellers is risky, as mentioned. You can buy somebody’s problem, or a lost or stolen (and permanently locked) Mac, or other messes.

FWIW, there was no 2016 iMac. It has to be a 2015 or 2017 iMac. Need more info.

Purchasing a used Mac is a task for very experienced and highly knowledgeable Mac users. For that matter so is selling one, and that Mac as not properly prepared for sale.

It may not be possible for you to fix it, and will remain useless to you or anyone else until it is so prepared.

If at all possible return it for a full cash refund.

+1 on den.thed's wise advice. Please return the computer if you cannot verify it was setup per the link he provided. If you don't you will live with a frustration that will not resolve until the computer is setup properly for a new owner (you) to own. We have seen hundreds of posts similar to yours and the answer is always the same.

If you cannot afford to buy a new Mac then buy a used Mac from a qualified source such as Apple's refurbished program or Macsales.com.

Never ever buy from:

• Ebay
• Amazon
• Craigslist
• Nextdoor
• Marketplace
• Local PC shop
• Friend, relative, aquantance.

Why, simple most of these are NOT setup for sale or give away properly and when they say they are refurbished they are NOT professionally refurbished with high quality parts!!!