Problem; you are trying to order a 3 oz copper board but the order form keeps telling you your trace/space is only 6mils.
This is not necessarily because you didn't route your board with big fat traces and sufficient spacing to keep from shorting, it is because your default setup has not bee updated to reflect your desired result.
In order to change this, you will need to do a couple of things.
First, select VIEW - USER PREFERENCES
Change net trace width to your desired minimum, default is .006in.
Change line width to that same desired minimum.
Click on the Routing and Spacing Rules tab in the User Preferences popup.
Change All Layers spacing to your new desired minimum spacing, default is 0.006in
Once this is done, you will not be able to route any wires smaller than whatever you have defined as your desired minimum trace/space rule.
Even more importantly, you will have the ability to order your 3oz copper board with minimum trace/space to support that, your pricing will reflect that wider trace/space as well.
PCB123 will not currently allow heavy copper weight for multi-layer boards, but your routing rules will support the heavier copper.
The workaround for this is to log into your sunstone account, order the board using the onequote order form instead of going through the pcb123 order process, then just upload your pcb123 file to the quote/order.