Hello, I dont really know if this is a scripting issue or building issue.
This is what happens when I add accessory to the character.
This is the hierarchy of the ODM gear model.
The Model Should be attached on the back of the character but its attaching on the head
I’m absolutely certain that the attachment must be strictly named exactly after the character’s attachment. Check again.
What do you mean? I dont get it sorry
The character has a set of attachments to certain body parts. Copy the attachment’s name and paste it over the name of the accessory’s attachment. It must match.
I have the attachments name 
Just like the body’s attachment name
Is there any tutorial I can follow? The developer wiki doesnt teach much and I cant find useful tutorials on youtube.
Remove all attachments from the accessory and re-add the BodyBackAttachment in first. It seems Accessories rely on using the first found attachment parented to Handle.
Thank you it works! 
Also one more thing;
My character doesnt moves now, its just stuck there.
How do I check what is causing that?
Check the Anchored property of the Handle part in the Accessory. Ensure it is false.