Reason: There are more than 1 PCB in the gerber file. We cannot confirm if you need the boards to be made in panel 

without v-cut lines/tab-routes or in separate single pcs or in panel with v-cut lines or tab-routes?

Suggestion:

A: Choose "panel by customer" when you place the order if your file is in panel design with v-cut lines or tab-routes.

B: If you need the boards to be made in panel without v-cut lines/tab-routes, please note it when place the order and choose "----" for route process.

C: If you need the boards to be shipped in separate single pcs, please make the boards in several gerber files and upload them one by one. The price will be cheaper.

It is also fine that you make them in 1 gerber file. Just note you need the boards to be shipped in separate single pcs when place the order.