How can you fix an acer laptop that says "No bootable device"?

That is an issue that is usually caused by one of three things. You either have a bad hard drive, a bad controller on your motherboard, or a faulty connection. I've dealt with all three. There isn't much you can do about a faulty motherboard. A bad connection is solved simply by putting the drive back in place. A bad hard drive could mean the loss of your data, as well as the cost of a new one (usually around $50 for a 1 terabyte drive, and much more for a solid state drive.) This doesn't include service fees, and data recovery programs can get extremely expensive.

Not sure myself

Hard drive error or it's not plugged in

That's a very technical question that will require a visit to the Geek Squad.