You need a permission plugin. I recommend Luckperms as it is pretty good in my opinion. You also have to setup the permissions right. If this is your first time, it can be confusing but I can try and help!
If you are going to use Luckperms, this is the command to give them the permission to /withdraw with the Coins plugin.
lp group <group name> permission set coins.withdraw true
make sure the <group name> is your group name (The group name that you want to give that permission to)!
For example, if my group name is default and I want to give it the permission, I would do this command:
lp group default permission set coins.withdraw true
Hope this helps you in any way! If it doesn't, reply to this and let me know the problem and I will try and help you with it.