Do you have a sales person when selling products? If so, the configuration discussed in this article may be very helpful to you in preventing the cashier from forgetting to input the Sales Person in each transaction. If this configuration is active, when the cashier doesn't enter the Sales Person in a sales transaction, the transaction cannot be processed. Details related to this configuration you can see the explanation below:
I. Configuration
Step 1. Go to the Setup menu, click more menu, and select module

Step 2. Go to the Sell menu and tick YES to 'Sales Person Required' and Save

II. Sell Menu

After the save was made, you are no longer allowed to use the Sell menu without inputting the Sales Person Information.
Sales Person Required in Sell Menu Tutorial Video
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You can enable the following configuration (Customer Required in Sell Menu) to secure that the cashier doesn't forget to enter the customer's name.
You can customize the Invoice number template, see the detailed steps here: Adding Prefix for Transaction Number
If you want to restrict users from creating new customer contacts on the Sell menu, you can restrict the access: Disable Add New Customer Button in Sell