Trying to wipe a locked Mac OS El Capitan
I’ve tried command/ R while turning on the computer but what comes up is a locked icon.
any help would be greatly appreciated
thanks
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That indicates the iMac has a firmware password lock enabled. You need to enter the firmware password in order to boot into any special modes. If you don't know the firmware password, then you will need to take the iMac to an Apple Store or an Apple Authorized Service Provider along with acceptable proof of purchase and personal ID to have Apple or the AASP remove the firmware lock. Since it appears you have a very old iMac, Apple and the AASP may at first refuse to help you. There are no other ways to remove a firmware lock. Here is an Apple article about the firmware lock:
Set a firmware password on your Mac - Apple Support
As long as the OS is still bootable on the internal drive, then you should be able to continue using the iMac as usual.
If you just purchased this used iMac, then return the iMac to the seller for a full refund since the seller/previous owner did not properly prepare the Mac for sale. Here is what the previous owner should have done prior to selling the Mac. This is just one of the dangers of purchasing a used Mac.
What to do before you sell, give away, or trade in your Mac - Apple Support