Is it mandatory to select a project while marking the bill as billable for a customer?

No, it is not mandatory to select a Project while marking the bill as Billable to a customer.

Here’s how you update your billing preferences:

• Navigate to Bills under the Sales module.
• Click +New.
• Fill in the necessary details such as Vendor Name, Bill# and Bill Date.
• Choose a customer with whom you wish to associate the bill.
• Click the Edit icon next to Non-Billable which is displayed under the Customer Details drop-down.
• Update your billing preferences in the pop-up that appears. Check the box next to Billable if you wish to bill the item.
• Click Update.