When an order or invoice cannot be created due to existing consecutive number, you should check the next number to take and change it.

On the device, you may go to Main Menu > Database > Sales/Invoices > press the gear icon > Sales > Next Order Number > Change on the suffix box > Done or Invoice > Next Invoice Number > change on the suffix box > Done

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On the web version, Database > Sales/Invoices > gear icon > Next Order Number or Next Invoice Number > Save

Click on Settings

Change the Next Order or Invoice numbers

On the device you should go to Database > Sales/Invoices > Settings > Sales orders > Nerxt order number

Database 

Sales/Invoices

Settings > Sales orders

Next order number 

Change the suffix then Done

Settings > Invoices

Next Invoice Number

Change the suffix then done

Now if you need to change the label on the PDF Invoice, tap on the invoice box and then done