@Override
public void writeToNBT(NBTTagCompound nbt) {
    super.writeToNBT(nbt);

    NBTTagList list = new NBTTagList();
    for (int i = 0; i < this.getSizeInventory(); ++i) {
        if (this.getStackInSlot(i) != null) {
            NBTTagCompound stackTag = new NBTTagCompound();
            stackTag.setByte("Slot", (byte) i);
            this.getStackInSlot(i).writeToNBT(stackTag);
            list.appendTag(stackTag);
        }
    }
    nbt.setTag("Items", list);

    if (this.hasCustomName()) {
        nbt.setString("CustomName", this.getCustomName());
    }
}

Thats my problem it does not want to have void and i am so confused it might be basic but im confused as hell on the WriteToNBT Method

writeToNBT is no longer a void method, instead, it is now a method using the object NBTTagCompound. This means that you need to return said object.

public NBTTagCompound writeToNBT(NBTTagCompound nbt) {
   //Do stuff with the gained nbt parameter.
   return nbt;
}