My brother gxl 5600 won't work. It was given to me - appears to be brand new or used once - still in the plastic. When I plug the foot pedal in - the light comes on, but no movement.

Hi Annie, It all has to do with your foot pedal. CHECK to make sure it's plugged into your machine properly.
IF it jiggles around in the socket, wrap a bit of tape around the rubber/plastic part that goes into the socket. I had to do that with one of my machines about a month after I got it...that was 10 years ago, still works great.
If these things don't work, then you may need a new foot pedal, or a new socket...or both.

THE link below will take you right to the page in the manual that explains the error. I hope the tape works for you:)
https://www.manualslib.com/manual/1266781/Brother-Nv-10a.html?page=183#manual

Try the following.

Presser foot up.
Rotate handwheel so needle up.
Remove top thread, spool, and bobbin.

Power off

Select straight stitch pattern 2.
Select stitch width 2.5 - needle position middle.
Set tension to 4.

Power on.

1. Press foot pedal.
2. If still knocks, remove needle.
3. Press foot pedal.
4. If still knocks, move bobbin winding spindle all the way right and back all the way left.
5. Press foot pedal.
6. If still knock, remove bobbin case (pg 47 in manual)
7. Press foot pedal.
8. if here and still knock... not sure what it is.

https://download.brother.com/welcome/doch100101/xm2701_2800om03enes.pdf

Might the plug connecting the Pedal be plugged into the machine wrong. Possibly turn the plug 180-degrees and replug.

you can take the sewing machine and foot pedal to a repair shop and have them check the foot pedal and they can tell you if you need a new foot pedal or if that one can be fixed.

the foot pedal may need to be checked to see if there is a electronic
board in it and it may need to be replaced.

Your new but stored machine has sat long enough that any oils from the factory have become gummy and or dried out. I would take it in and have the pros service it. But for now you might try....

Turning the hand wheel by hand while you press on the pedal. This MAY free up the machine enough for the motor to take over.

grant514 Has pretty much hit the nail on the head but the RF suppressor is located in the motor housing, not the foot pedal. It bridged the terminals where the foot pedal connects. Removing this capacitor resolved the same issue in my machine and given the absence of radio receiving equipment in my home now I have had no ill effects.

It most likely is your foot pedal. Unplug your foot pedal and start the machine without it. If it works properly without the foot pedal plugged in then your foot pedal needs repaired or replaced. This worked for us with the NX400 machine and I am sure the NX450 is closely similar. Don't bother to buy a Manual if you didn't already get one with the machine. A Service manual and parts manual is best; But just so you know they don't show the error codes unless you are a qualified and authorized repair person for the machines. I got my info from the repair man where my wife buys and repairs her machines. I am an electronic technician and mechanic so I understand how this system works. I will be repairing my wife's machines from now on.

You will have better luck if you go to a local sewing or fabrics shop. The smaller the better. They will know how to get this for you. Smaller shops usually mean older people there who know what their talking about and they've been at that store since it opened. just guessing here, lol. But seriouslly, if they can't find it for you then check on ebay. They definately will.