We have an Optra M412 that has a 921 Service fuser error. We don't know where to find the fuser

Service Fuser Error appears when the fuser does not warm to the required temperature. For this error on this Lexmark model, the manual states you should power off the printer and power it back on, and if it is not plugged into a wall power point, change it to a power point mounted directly on the wall. Personally, I never got a Lexmark printer to work doing this, but this is in the manual. If this won't help, and the printer is under warranty, you should log it in with the servicing company.

You can find the fuser at the back of the printer, remove the cover and there it is. It holds in two screws and a power connection. If the printer is out of warranty, your next option is to change the fuser which costs around $200, so I think you would be better off getting a new printer unless you find a second hand one on Ebay.